Preparation of a synthetic quartz glass grain

ABSTRACT

One aspect relates to a process for the preparation of a quartz glass grain, including providing a silicon dioxide granulate from a pyrogenically produced silicon dioxide powder, making a glass melt out of silicon dioxide granulate, making a quartz glass body out of at least part of the glass melt and reducing the size of the quartz glass body to obtain the quartz glass grain. One aspect further relates to a quartz glass grain which is obtainable by this process. One aspect further relates to opaque quartz glass bodies, which are obtainable by further processing of the quartz glass grain.

The invention relates to a process for the preparation of a quartz glass grain comprising the process steps i.) Providing a silicon dioxide granulate from a pyrogenically produced silicon dioxide powder, ii.) Making a glass melt out of the silicon dioxide granulate, iii.) Making a quartz glass body out of at least a part of the glass melt and iv.) Reducing the size of the quartz glass body to obtain a quartz glass grain. Furthermore, the invention relates to a quartz glass grain obtainable by this process. Furthermore, the invention relates to opaque quartz glass bodies which are obtainable by further processing of the quartz glass grain.

BACKGROUND OF THE INVENTION

Quartz glass, quartz glass products and products which contain quartz glass are known. Likewise, various processes for the preparation of quartz glass, quartz glass bodies and grains of quartz glass are already known. Nonetheless, considerable efforts are still being made to identify preparation processes by which quartz glass of even higher purity, i.e. absence of impurities, can be prepared. In many areas of application of quartz glass and its processed products, high demands are made, for example in terms of purity. This is the case, inter alia, for quartz glass which is provided for an application in production steps in the fabrication of semiconductors. Here, every impurity of a glass body can potentially lead to defects in the semiconductor and thus to rejects in the fabrication. The varieties of high purity quartz glass which are employed in these processes are therefore laborious to prepare. These are valuable.

Furthermore, there is a market requirement for the above mentioned high purity quartz glass and products derived therefrom at low price. Therefore, it is an aspiration to be able to offer high purity quartz glass at a lower price than before. In this connection, both more cost-efficient preparation processes as well as cheaper sources of raw materials are sought.

Known processes for the preparation of quartz glass grains comprise melting silicon dioxide, making quartz glass bodies out of the melt and reducing the size of the quartz glass body to a grain. Impurities of the glass body made at the outset can lead to a failure of an opaque glass body made from the grain under load, in particular at high temperatures, or can preclude its use for a particular purpose. Impurities in the raw materials in the opaque glass body can also be released and transferred to the treated semiconductor components. This is the case, for example, in etching processes and leads to rejects in the semiconductor billets. A common problem associated with known preparation processes is therefore an inadequate quality of the purity of quartz glass bodies.

A further aspect relates to raw materials efficiency. It appears advantageous to input quartz glass and raw materials, which accumulate elsewhere as side products, into a preferably industrial process for quartz glass products, rather than employ these side products as filler, e.g. in construction or to dispose of them as rubbish at a cost.

These side products are often separated off as fine dust in filters. The fine dust brings further problems, in particular in relation to health, work safety and handling

Objects

An object of the present invention is to at least partially overcome one or more of the disadvantages present in the state of the art.

It is a further object of the invention to provide opaque components with a long lifetime. The term opaque components in particular is to be understood to include devices which can be employed in reactors for chemical and/or physical treatment steps.

It is a further object of the invention to provide opaque components which are suitable in particular for specific treatment steps in the fabrication of semiconductors, in particular in the preparation of wafers. Examples of such a specific treatment step is plasma etching, chemical etching and plasma doping.

It is a further object of the invention to provide opaque components which are free of bubbles or have a low content of bubbles.

It is a further object of the invention to provide opaque components which have a high opacity.

It is a further object of the invention to provide opaque components which have a high contour accuracy. In particular, it is an object of the invention to provide opaque components which do not deform at high temperatures.

In particular, it is an object of the invention to provide opaque components which are form stable, even when formed with large size.

It is a further object of the invention to provide opaque components which are tear-proof and break-proof.

It is a further object of the invention to provide opaque components which are efficient to prepare.

It is a further object of the invention to provide opaque components which are cost-efficient to prepare.

It is a further object of the invention to provide opaque components, the preparation of which does not require long further processing steps, for example tempering.

It is a further object of the invention to provide opaque components which have a high thermal shock resistance. It is in particular an object of the invention to provide opaque components which with large thermal fluctuations exhibit only little thermal expansion.

It is a further object of the invention to provide opaque components with a high hardness.

It is a further object of the invention to provide opaque components which have a high purity and low contamination with foreign atoms. The term foreign atoms is employed to mean constituents at a concentration of less than 10 ppm which are not purposefully introduced.

It is a further object of the invention to provide opaque components which contain a low content of dopant materials.

It is a further object of the invention to provide opaque components which have a high material homogeneity. The material homogeneity is a measure of the uniformity of the distribution of the elements and compounds, in particularly of OH, chlorine, metals, in particular aluminium, alkali earth metals, refractory metals and dopant materials, contained in the opaque component.

It is a further object of the invention to provide strongly reflecting components made of quartz glass. It is a further object of the invention to provide uniformly reflecting components made of quartz glass.

It is a further object of the invention to provide components made of quartz glass which have a low thermal conduction.

It is a further object of the invention to provide components made of glass which display low stresses under thermal load.

It is a further object of the invention to provide thin components made of quartz glass. It is a further object of the invention to provide light components made of quartz glass. It is a further object of the invention to provide components made of glass, which components have a long working life.

It is a further object of the invention to provide large components made of quartz glass.

It is a further object of the invention to provide components made of quartz glass which have a low marbling. It is a further object of the invention to provide components made of quartz glass which do not have any transmission speckles.

It is a further object of the invention to provide components made of quartz glass which can be etched easily.

It is a further object of the invention to provide components made of quartz glass which are uniformly opaque and simultaneously densely sintered.

It is a further object of the invention to provide cost-efficient melting crucibles made of synthetic quartz glass.

It is a further object of the invention to provide bedding materials made of synthetic quartz glass for use in metal welding methods. It is in particular an object of the invention to provide bedding materials made of synthetic quartz glass for use in metal welding methods which avoid contaminations.

It is a further object of the invention to provide an opaque quartz glass body which is suitable for use in quartz glass components and solves at least partly at least one, preferably several, of the above mentioned objects.

It is a further object to provide an opaque quartz glass body which has a high homogeneity over the entire length of the quartz glass body.

In particular, it is a further object of the invention to provide an opaque quartz glass body which has a high material homogeneity over the entire length of the opaque quartz glass body.

In particular, it is a further object of the invention to provide an opaque quartz glass body which has a high optical homogeneity over the entire length of the opaque quartz glass body.

It is a further object of the invention to provide an opaque quartz glass body, which comprises quartz glass of a defined composition.

It is a further object of the invention to provide an opaque quartz glass body with high purity.

It is a further object to provide an opaque quartz glass body efficiently and cost-efficiently.

It is a further object of the invention to provide a cost-efficient quartz glass grain.

It is a further object of the invention to provide a synthetic quartz glass grain.

It is a further object of the invention to provide an amorphous quartz glass grain.

It is a further object of the invention to provide a high-density quartz glass grain.

It is a further object of the invention to provide a quartz glass grain, the definition of which is chemically precise.

It is a further object of the invention to provide a quartz glass grain which has a high material homogeneity.

It is a further object of the invention to provide a quartz glass grain which has a high purity.

It is a further object of the invention to provide a quartz glass grain. with a low melting point.

It is a further object of the invention to provide a quartz glass grain which has a uniform melting behaviour. It is in particular an object of the invention to provide a quartz glass grain which can be melted quickly to form a homogeneous quartz glass melt.

It is a further object of the invention to provide a quartz glass grain with a good handlability.

It is a further object of the invention to provide a quartz glass grain which can be easily stored, transported and conveyed.

It is a further object of the invention to provide a quartz glass grain which is free from bubbles. It is a further object of the invention to provide a quartz glass grain which can form bubble free opaque quartz glass bodies.

It is a further object of the invention to provide a quartz glass grain from which homogeneous green bodies can be formed. It is a further object of the invention to provide a quartz glass grain from which green bodies with high green body strength can be produced without addition of binders. It is a further object of the invention to provide a quartz glass grain from which green bodies with high form stability can be produced

It is a further object of the invention to provide a process by which opaque quartz glass bodies can be prepared by which at least part of the above described objects is at least partly solved.

It is a further object of the invention to provide a process by which opaque quartz glass bodies can be more simply prepared.

It is a further object of the invention to provide a process by which opaque quartz glass bodies can be formed with a high speed.

It is a further object of the invention to provide a process by which opaque quartz glass bodies can be prepared with a low reject rate.

PREFERRED EMBODIMENTS OF THE INVENTION

A contribution to at least partially fulfilling at least one of the aforementioned objects is made by the independent claims The dependent claims provide preferred embodiments which contribute to at least partially fulfilling at least one of the objects.

-   |1| A process for the preparation of a quartz glass grain comprising     the process steps:

1 i.) Providing a silicon dioxide granulate, wherein the silicon dioxide granulate has been prepared from pyrogenically produced silicon dioxide powder;

-   -   ii.) Making a glass melt out of the silicon dioxide granulate in         a melting crucible;     -   iii.) Making a quartz glass body out of at least part of the         glass melt;     -   iv.) Reducing the size of the quartz glass body to obtain a         quartz glass grain.

-   |2| The process according to embodiment |1|, wherein the melting     crucible in step ii.) has at least one inlet and an outlet and     wherein the inlet is arranged above the outlet.

-   |3| The process according to one of the preceding embodiments,     wherein the reduction in size in step iv.) takes place by high     voltage discharge pulses.

-   |4| The process according to one of the preceding embodiments,     wherein the quartz glass grain has at least one of the following     features:     -   I/an OH content of less than 500 ppm;     -   II/a chlorine content of less than 60 ppm;     -   III/an aluminium content of less than 200 ppb;     -   IV/a BET surface area of less than 1 m²/g;     -   V/a bulk density in a range from 1.1 to 1.4 g/cm³.     -   VI/a particle size D₅₀ for melting in a range from 50 to 500 μm;     -   VII/a particle size D₅₀ for the slurry in a range from 0.5 to 5         μm;     -   VIII/a metal content of metals different to aluminium of less         than 2 ppm;     -   IXviscosity (p=1013 hPa) in a range from log₁₀ (η(1250°         C.)/dPas)=11.4 to log₁₀ (η(1250° C.)/dPas)=12.9 or log₁₀         (η(1300° C.)/dPas)=11.1 to log₁₀ (η(1300° C.)/dPas)=12.2 or         log₁₀ (η(1350° C.)/dPas)=10.5 to log₁₀ (η(1350° C.)/dPas)=11.5;     -   wherein the ppm and ppb are each based on the total weight of         the quartz glass grain.

-   |5| The process according to one of the preceding embodiments,     wherein the melt energy in process step ii.) is transferred to the     silicon dioxide granulate via a solid surface.

-   |6| The process according to one of the preceding embodiments,     wherein the silicon dioxide granulate has at least one of the     following features:     -   A) a BET surface area in a range from 20 to 50 m²/g;     -   B) a mean particle size in a range from 50 to 500 μm;     -   C) a bulk density in a range from 0.5 to 1.2 g/cm³.     -   D) a carbon content of less than 50 ppm;     -   E) an aluminium content of less than 200 ppb;     -   F) a tamped density in a range from 0.7 to 1.0 g/cm³;     -   G) a pore volume in a range from 0.1 to 2.5 mL/g;     -   H) an angle of repose in a range from 23 to 26°,     -   I) a particle size distribution D₁₀ in a range from 50 to 150         μm;     -   J) a particle size distribution D₅₀ in a range from 150 to 300         μm;     -   K) a particle size distribution D₉₀ in a range from 250 to 620         μm,     -   wherein the ppm and ppb are each based on the total weight of         the silicon dioxide granulate.

-   |7| The process according to one of the preceding embodiments,     wherein the silicon dioxide powder can be prepared from a compound     selected from the group consisting of siloxanes, silicon alkoxides     and silicon halides.

-   |8| A quartz glass grain obtainable by a process according to one of     the preceding embodiments.

-   |9| The quartz glass grain according to embodiment |8|, having at     least one of the following features:     -   I/an OH content of less than 500 ppm;     -   II/a chlorine content of less than 60 ppm;     -   III/an aluminium content of less than 200 ppb;     -   IV/a BET surface area of less than 1 m²/g;     -   V/a bulk density in a range from 1.1 to 1.4 g/cm³.     -   VI/a particle size D₅₀ for melting in a range from 50 to 500 μm;     -   VII/a particle size D₅₀ for the slurry in a range from 0.5 to 5         μm;     -   VIII/a metal content of metals different to aluminium of less         than 2 ppm;     -   IX/viscosity (p=1013 hPa) in a range from log₁₀ (η(1250°         C.)/dPas)=11.4 to log₁₀ (η(1250° C.)/dPas)=12.9 or log₁₀         (η(1300° C.)/dPas)=11.1 to log₁₀ (η(1300° C.)/dPas)=12.2 or         log₁₀ (η(1350° C.)/dPas)=10.5 to log₁₀ (η(1350° C.)/dPas)=11.5;     -   wherein the ppm and ppb are each based on the total weight of         the quartz glass grain.

-   |10| A process for the preparation of an opaque formed body     comprising the following steps:     -   (1) Providing a quartz glass grain according to one of the         embodiments |8| or |9|; and     -   (2) Processing the quartz glass grain to an opaque formed body.

-   |11| The process according to embodiment |10|, wherein the process     comprises a heating, preferably a sintering.

-   |12| An opaque formed body obtainable by a process according to one     of embodiments |10| or |11|.

-   |13| The opaque formed body according to embodiment |12|, wherein     the opaque formed body has at least one of the following features:     -   I] an opacity of more than 5;     -   II] a BET surface area of less than 1 m²/g;     -   III] a cylindrical form;     -   IV] a sheet;     -   V] an OH content of less than 500 ppm;     -   VI] a chlorine content of less than 60 ppm;     -   VII] an aluminium content of less than 200 ppb;     -   VIII] an ODC content of less than 5*10¹⁸/cm³;     -   IX] a carbon content of less than 5 ppm;     -   X] a total metal content of metals different to aluminium of         less than 2 ppm;     -   wherein the ppm and ppb are each based on the total weight of         the formed body.

-   |14| A use of a quartz glass grain for improving the green body     strength of binder-free green bodies made of quartz glass, wherein     the quartz glass grain has the following features:     -   I/an OH content of less than 500 ppm;     -   II/a chlorine content of less than 60 ppm;     -   III/an aluminium content of less than 200 ppb;     -   wherein the ppm and ppb are each based on the total weight of         the quartz glass grain.

-   |15| A use of a quartz glass grain for preparing opaque products     comprising quartz glass, wherein the quartz glass grain has the     following features:     -   I/an OH content of less than 500 ppm;     -   II/a chlorine content of less than 60 ppm;     -   III/an aluminium content of less than 200 ppb;     -   wherein the ppm and ppb are each based on the total weight of         the quartz glass grain.

General

In the present description disclosed ranges also include the boundary values. A disclosure of the form “in the range from X to Y” in relation to a parameter A therefore means that A can take the values X, Y and values in between X and Y. Ranges bounded on one side of the form “up to Y” for a parameter A mean correspondingly the value Y and those less than Y.

DETAILED DESCRIPTION OF THE INVENTION

A first aspect of the present invention is a process for the preparation of a quartz glass grain comprising the process steps:

-   -   i.) Providing a silicon dioxide granulate, wherein the silicon         dioxide granulate has been prepared from pyrogenically produced         silicon dioxide powder;     -   ii.) Making a glass melt out of the silicon dioxide granulate in         a melting crucible;     -   iii.) Making a quartz glass body out of at least part of the         glass melt.     -   iv.) Reducing the size of the quartz glass body from step iii.)         to obtain the quartz glass grain.

Step i.)

According to a preferred embodiment of the first aspect of the invention, the provision of the silicon dioxide granulate comprises the following process steps:

I. Providing a pyrogenically produced silicon dioxide powder; and

-   -   II. Processing the silicon dioxide powder to obtain a silicon         dioxide granulate, wherein the silicon dioxide granulate has a         greater particle diameter than the silicon dioxide powder.

A powder means particles of a dry solid material with a primary particle size in the range from 1 to less than 100 nm.

The silicon dioxide granulate can be obtained by granulating silicon dioxide powder. A silicon dioxide granulate commonly has a BET surface area of 3 m²/g or more and a density of less than 1.5 g/cm³. Granulating means transforming powder particles into granules. During granulation, clusters of multiple silicon dioxide powder particles, i.e. larger agglomerates, form which are referred to as “silicon dioxide granules”. These are often also called “silicon dioxide granulate particles” or “granulate particles”. Collectively, the granules form a granulate, e.g. the silicon dioxide granules form a “silicon dioxide granulate”. The silicon dioxide granulate has a larger particle diameter than the silicon dioxide powder.

The granulation procedure, for transforming a powder into a granulate, will be described in more detail later.

Silicon dioxide grain in the present context means silicon dioxide particles which are obtainable by reduction in size of a silicon dioxide body, in particular of a quartz glass body. A silicon dioxide grain commonly has a density of more than 1.2 g/cm³, for example in a range from 1.2 to 2.2 g/cm³, and particularly preferably of about 2.2 g/cm³. Furthermore, the BET surface area of a silicon dioxide grain is preferably generally less than 1 m²/g, determined according to DIN ISO 9277:2014-01.

In principle, all silicon dioxide particles which are considered to be suitable by the skilled man can be selected. Preferred are silicon dioxide granulate and silicon dioxide grain.

Particle diameter or particle size mean the diameter of a particle, given as the “area equivalent circular diameter x_(Ai)” according to the formula

${x_{Ai} = \sqrt{\frac{4A_{i}}{\pi}}},$

wherein Ai stands for the surface area of the observed particle by means of image analysis. Suitable methods for the measurement are for example ISO 13322-1:2014 or ISO 13322-2:2009. Comparative disclosures such as “greater particle diameter” always means that the values being compared are measured with the same method.

Silicon Dioxide Powder

In the context of the present invention, synthetic silicon dioxide powder, namely pyrogenically produced silicon dioxide powder, is used.

The silicon dioxide powder can be any silicon dioxide powder which has at least two particles. As preparation process, any process which the skilled man considers to be prevalent in the art and suitable can be used.

According to a preferred embodiment of the present invention, the silicon dioxide powder is produced as side product in the preparation of quartz glass, in particular in the preparation of so called “soot bodies”. Silicon dioxide from such a source is often also called “soot dust”.

A preferred source for the silicon dioxide powder are silicon dioxide particles which are obtained from the synthetic preparation of soot bodies by application of flame hydrolysis burners. In the preparation of a soot body, a rotating carrier tube with a cylinder jacket surface is moved back and forth along a row of burners. Flame hydrolysis burners can be fed with oxygen and hydrogen as burner gases as well as the raw materials for making silicon dioxide primary particles. The silicon dioxide primary particles preferably have a primary particle size of up to 100 nm. The silicon dioxide primary particles produced by flame hydrolysis aggregate or agglomerate to form silicon dioxide particles with particle sizes of about 9 μm (DIN ISO 13320:2009-1). In the silicon dioxide particles, the silicon dioxide primary particles are identifiable by their form by scanning electron microscopy and the primary particle size can be measured. A portion of the silicon dioxide particles are deposited on the cylinder jacket surface of the carrier tube which is rotating about its longitudinal axis. In this way, the soot body is built up layer by layer. Another portion of the silicon dioxide particles are not deposited on the cylinder jacket surface of the carrier tube, rather they accumulate as dust, e.g. in a filter system. This other portion of silicon dioxide particles make up the silicon dioxide powder, often also called “soot dust”. In general, the portion of the silicon dioxide particles which are deposited on the carrier tube is greater than the portion of silicon dioxide particles which accumulate as soot dust in the context of soot body preparation, based on the total weight of the silicon dioxide particles.

These days, soot dust is generally disposed of as waste in an onerous and costly manner, or used as filler material without adding value, e.g. in road construction, as additive in the dyes industry, as a raw material for the tiling industry and for the preparation of hexafluorosilicic acid, which is employed for restoration of construction foundations. In the case of the present invention, it is a suitable raw material and can be processed to obtain a high-quality product.

Silicon dioxide prepared by flame hydrolysis is normally called pyrogenic silicon dioxide. Pyrogenic silicon dioxide is normally available in the form of amorphous silicon dioxide primary particles or silicon dioxide particles.

According to a preferred embodiment, the silicon dioxide powder can be prepared by flame hydrolysis out of a gas mixture. In this case, silicon dioxide particles are also created in the flame hydrolysis and are taken away before agglomerates or aggregates form. Here, the silicon dioxide powder, previously referred to as soot dust, is the main product.

Suitable raw materials for creating the silicon dioxide powder are preferably siloxanes, silicon alkoxides and inorganic silicon compounds. Siloxanes means linear and cyclic polyalkylsiloxanes. Preferably, polyalkylsiloxanes have the general formula

Si_(p)O_(p)R_(2p),

-   -   wherein p is an integer of at least 2, preferably from 2 to 10,         particularly preferably from 3 to 5, and R is an alkyl group         with 1 to 8 C-atoms, preferably with 1 to 4 C-atoms,         particularly preferably a methyl group.

Particularly preferred are siloxanes selected from the group consisting of hexamethyldisiloxane, hexamethylcyclotrisiloxane (D3), octamethylcyclotetrasiloxane (D4) and decamethylcyclopentasiloxane (D5) or a combination of two or more thereof. If the siloxane comprises D3, D4 and D5, then D4 is preferably the main component. The main component is preferably present in an amount of at least 70 wt.-%, preferably of at least 80 wt.-%, for example of at least 90 wt.-% or of at least 94 wt.-%, particularly preferably of at least 98 wt.-%, in each case based on the total amount of the silicon dioxide powder. Preferred silicon alkoxides are tetramethoxysilane and methyl-trimethoxysilane. Preferred inorganic silicon compounds as raw material for silicon dioxide powder are silicon halides, silicates, silicon carbide and silicon nitride. Particularly preferred inorganic silicon compounds as raw material for silicon dioxide powder are silicon tetrachloride and trichlorosilane.

According to a preferred embodiment, the silicon dioxide powder can be prepared from a compound selected from the group consisting of siloxanes, silicon alkoxides and silicon halides.

Preferably, the silicon dioxide powder can be prepared from a compound selected from the group consisting of hexamethyldisiloxane, hexamethylcyclotrisiloxane, octamethylcyclotetrasiloxane and decamethylcyclopentasiloxane, tetramethoxysilane and methyltrimethoxysilane, silicon tetrachloride and trichlorosilane or a combination of two or more thereof, for example out of silicon tetrachloride and octamethylcyclotetrasiloxane, particularly preferably out of octamethylcyclotetrasiloxane.

For making silicon dioxide out of silicon tetrachloride by flame hydrolysis, various parameters are significant. A preferred composition of a suitable gas mixture comprises an oxygen content in the flame hydrolysis in a range from 25 to 40 vol.-%. The content of hydrogen can be in a range from 45 to 60 vol.-%. The content of silicon tetrachloride is preferably 5 to 30 vol.-%, all of the afore mentioned vol.-% being based on the total volume of the gas flow. Further preferred is a combination of the above mentioned volume proportions for oxygen, hydrogen and SiCl₄. The flame in the flame hydrolysis preferably has a temperature in a range from 1500 to 2500° C., for example in a range from 1600 to 2400° C., particularly preferably in a range from 1700 to 2300° C. Preferably, the silicon dioxide primary particles created in the flame hydrolysis are taken away as silicon dioxide powder before agglomerates or aggregates form.

According to a preferred embodiment of the first aspect of the invention, the silicon dioxide powder has at least one, for example at least two or at least three or at least four, particularly preferably at least five of the following features:

-   -   a. a BET surface area in a range from 20 to 60 m²/g, for example         from 25 to 55 m²/g, or from 30 to 50 m²/g, particularly         preferably from 20 to 40 m²/g,     -   b. a bulk density 0.01 to 0.3 g/cm³, for example in the range         from 0.02 to 0.2 g/cm³, preferably in the range from 0.03 to         0.15 g/cm³, further preferably in the range from 0.1 to 0.2         g/cm³ or in the range from 0.05 to 0.1 g/cm³.     -   c. a carbon content of less than 50 ppm, for example of less         than 40 ppm or of less than 30 ppm, particularly preferably in a         range from 1 ppb to 20 ppm;     -   d. a chlorine content of less than 200 ppm, for example of less         than 150 ppm or of less than 100 ppm, particularly preferably in         a range from 1 ppb to 80 ppm;     -   e. an aluminium content of less than 200 ppb, for example in the         range from 1 to 100 ppb, particularly preferably in the range         from 1 to 80 ppb;     -   f. a total content of metals different to aluminium of less than         5 ppm, for example of less than 2 ppm, particularly preferably         in a range from 1 ppb to 1 ppm;     -   g. at least 70 wt.-% of the powder particles have a primary         particle size in a range from 10 to less than 100 nm, for         example in the range from 15 to less than 100 nm, particularly         preferably in the range from 20 to less than 100 nm;     -   h. a tamped density in a range from 0.001 to 0.3 g/cm³, for         example in the range from 0.002 to 0.2 g/cm³ or from 0.005 to         0.1 g/cm³, preferably in the range from 0.01 to 0.06 g/cm³, and         preferably in the range from 0.1 to 0.2 g/cm³, or in the range         of from 0.15 to 0.2 g/cm³;     -   i. a residual moisture content of less than 5 wt.-%, for example         in the range from 0.25 to 3 wt.-%, particularly preferably in         the range from 0.5 to 2 wt.-%;     -   j. a particle size distribution D₁₀ in the range from 1 to 7 μm,         for example in the range from 2 to 6 μm or in the range from 3         to 5 μm, particularly preferably in the range from 3.5 to 4.5         μm;     -   k. a particle size distribution D₅₀ in the range from 6 to 15         μm, for example in the range from 7 to 13 μm or in the range         from 8 to 11 μm, particularly preferably in the range from 8.5         to 10.5 μm;     -   l. a particle size distribution D₉₀ in the range from 10 to 40         μm, for example in the range from 15 to 35 μm, particularly         preferably in the range from 20 to 30 μm;     -   wherein the wt.-%, ppm and ppb are each based on the total         weight of the silicon dioxide powder.

The silicon dioxide powder contains silicon dioxide. Preferably, the silicon dioxide powder contains a proportion of silicon dioxide of more than 95 wt.-%, for example more than 98 wt.-% or more than 99 wt.-%. or more than 99.9 wt.-%, in each case based on the total weight of the silicon dioxide powder. Particularly preferably, the silicon dioxide powder contains a proportion of silicon dioxide of more than 99.99 wt.-%, based on the total weight of the silicon dioxide powder.

Preferably, the silicon dioxide powder has a metal content of metals different from aluminium of less than 5 ppm, for example of less than 2 ppm, particularly preferably of less than 1 ppm, in each case based on the total weight of the silicon dioxide powder. Often however, the silicon dioxide powder has a content of metals different to aluminium of at least 1 ppb. Such metals are for example sodium, lithium, potassium, magnesium, calcium, strontium, germanium, copper, molybdenum, tungsten, titanium, iron and chromium. These can be present for example in elemental form, as an ion, or as part of a molecule or of an ion or of a complex.

Preferably, the silicon dioxide powder has a total content of further constituents of less than 30 ppm, for example of less than 20 ppm, particularly preferably of less than 15 ppm, the ppm in each case being based on the total weight of the silicon dioxide powder. Often however, the silicon dioxide powder has a content of further constituents of at least 1 ppb. Further constituents means all constituents of the silicon dioxide powder which do not belong to the following group: silicon dioxide, chlorine, aluminium, OH-groups.

In the present context, reference to a constituent, when the constituent is a chemical element, means that it can be present as element or as an ion or in a compound or a salt. For example the term “aluminium” includes in addition to metallic aluminium, also aluminium salts, aluminium oxides and aluminium metal complexes. For example, the term “chlorine” includes, in addition to elemental chlorine, chlorides such as sodium chloride and hydrogen chloride. Often, the further constituents are present in the same aggregate state as the material in which they are contained.

In the present context, in the case where a constituent is a chemical compound or a functional group, reference to the constituent means that the constituent can be present in the form disclosed, as a charged chemical compound or as derivative of the chemical compound. For example, reference to the chemical material ethanol includes, in addition to ethanol, also ethanolate, for example sodium ethanolate. Reference to “OH-group” also includes silanol, water and metal hydroxides. For example, reference to derivate in the context of acetic acid also includes acetic acid ester and acetic acid anhydride.

Preferably, at least 70% of the powder particles of the silicon dioxide powder, based on the number of powder particles, have a primary particle size of less than 100 nm, for example in the range from 10 to 100 nm or from 15 to 100 nm, and particularly preferably in the range from 20 to 100 nm. The primary particle size is measured by dynamic light scattering according to ISO 13320:2009-10.

Preferably at least 75% of the powder particles of the silicon dioxide powder, based on the number of powder particles, have a primary particle size of less than 100 nm, for example in the range from 10 to 100 nm or from 15 to 100 nm, and particularly preferably in the range from 20 to 100 nm.

Preferably, at least 80% of the powder particles of the silicon dioxide powder, based on the number of powder particles, have a primary particle size of less than 100 nm, for example in the range from 10 to 100 nm or from 15 to 100 nm, and particularly preferably in the range from 20 to 100 nm.

Preferably, at least 85% of the powder particles of the silicon dioxide powder, based on the number of powder particles, have a primary particle size of less than 100 nm, for example in the range from 10 to 100 nm or from 15 to 100 nm, and particularly preferably in the range from 20 to 100 nm.

Preferably, at least 90% of the powder particles of the silicon dioxide powder, based on the number of powder particles, have a primary particle size of less than 100 nm, for example in the range from 10 to 100 nm or from 15 to 100 nm, and particularly preferably in the range from 20 to 100 nm.

Preferably, at least 95% of the powder particles of the silicon dioxide powder, based on the number of powder particles, have a primary particle size of less than 100 nm, for example in the range from 10 to 100 nm or from 15 to 100 nm, and particularly preferably in the range from 20 to 100 mm

Preferably, the silicon dioxide powder has a particle size D₁₀ in the range from 1 to 7 μm, for example in the range from 2 to 6 μm or in the range from 3 to 5 μm, particularly preferably in the range from 3.5 to 4.5 μm. Preferably, the silicon dioxide powder has a particle size D₅₀ in the range from 6 to 15 μm, for example in the range from 7 to 13 μm or in the range from 8 to 11 μm, particularly preferably in the range from 8.5 to 10.5 μm. Preferably, the silicon dioxide powder has a particle size D₉₀ in the range from 10 to 40 μm, for example in the range from 15 to 35 μm, particularly preferably in the range from 20 to 30 μm.

Preferably, the silicon dioxide powder has a specific surface area (BET-surface area) in a range from 20 to 60 m²/g, for example from 25 to 55 m²/g, or from 30 to 50 m²/g, particularly preferably from 20 to 40 m²/g. The BET surface area is determined according to the method of Bmnauer, Emmet and Teller (BET) by means of DIN 66132 which is based on gas absorption at the surface to be measured.

Preferably, the silicon dioxide powder has a pH value of less than 7, for example in the range from 3 to 6.5 or from 3.5 to 6 or from 4 to 5.5, particularly preferably in the range from 4.5 to 5. The pH value can be determined by means of a single rod measuring electrode (4% silicon dioxide powder in water).

The silicon dioxide powder preferably has the feature combination a./b./c. or a./b./f. or a./b./g., further preferred the feature combination a./b./c./f. or a./b./c./g. or a./b./f./g., further preferably the feature combination a./b./c./f./g.

The silicon dioxide powder preferably has the feature combination a./b./c., wherein the BET surface area is in a range from 20 to 40 m²/g, the bulk density is in a range from 0.05 to 0.3 g/mL and the carbon content is less than 40 ppm.

The silicon dioxide powder preferably has the feature combination a./b./f., wherein the BET surface area is in a range from 20 to 40 m²/g, the bulk density is in a range from 0.05 to 0.3 g/mL and the total content of metals which are different to aluminium is in a range from 1 ppb to 1 ppm.

The silicon dioxide powder preferably has the feature combination a./b./g., wherein the BET surface area is in a range from 20 to 40 m²/g, the bulk density is in a range from 0.05 to 0.3 g/mL and at least 70 wt. % of the powder particles have a primary particle size in a range from 20 to less than 100 nm.

The silicon dioxide powder further preferably has the feature combination a./b./c./f., wherein the BET surface area is in a range from 20 to 40 m²/g, the bulk density is in a range from 0.05 to 0.3 g/mL, the carbon content is less than 40 ppm and the total content of metals which are different to aluminium is in a range from 1 ppb to 1 ppm.

The silicon dioxide powder further preferably has the feature combination a./b./c./g., wherein the BET surface area is in a range from 20 to 40 m²/g, the bulk density is in a range from 0.05 to 0.3 g/mL, the carbon content is less than 40 ppm and at least 70 wt. % of the powder particles have a primary particle size in a range from 20 to less than 100 nm.

The silicon dioxide powder further preferably has the feature combination a./b./f./g., wherein the BET surface area is in a range from 20 to 40 m²/g, the bulk density is in a range from 0.05 to 0.3 g/mL, the total content of metals which are different to aluminium is in a range from 1 ppb to 1 ppm and at least 70 wt. % of the powder particles have a primary particle size in a range from 20 to less than 100 nm.

The silicon dioxide powder has particularly preferably the feature combination a./b./c./f./g., wherein the BET surface area is in a range from 20 to 40 m²/g, the bulk density is in a range from 0.05 to 0.3 g/mL, the carbon content is less than 40 ppm, the total content of metals which are different to aluminium is in a range from 1 ppb to 1 ppm and at least 70 wt. % of the powder particles have a primary particle size in a range from 20 to less than 100 nm.

Step II.

According to a preferred embodiment of the first aspect of the invention, silicon dioxide powder is processed in step II to obtain a silicon dioxide granulate, wherein the silicon dioxide granulate has a greater particle diameter than the silicon dioxide powder. For this purpose, any processes known to the skilled man that lead to an increase in the particle diameter are suitable.

The silicon dioxide granulate has a particle diameter which is greater than the particle diameter of the silicon dioxide powder. Preferably, the particle diameter of the silicon dioxide granulate is in a range from 500 to 50,000 times as great as the particle diameter of the silicon dioxide powder, for example 1,000 to 10,000 times as great, particularly preferably 2,000 to 8,000 times as great.

Preferably, at least 90% of the silicon dioxide granulate provided in step i.) is made up of pyrogenically produced silicon dioxide powder, for example at least 95 wt.-% or at least 98 wt.-%, particularly preferably at least 99 wt.-% or more, in each case based on the total weight of the silicon dioxide granulate.

According to a preferred embodiment of the first aspect of the invention, the silicon dioxide granulate employed has at least one, preferably at least two or at least three or at least four, particularly preferably all of the following features:

-   -   A) a BET surface area in the range from 20 m²/g to 50 m²/g;     -   B) a mean particle size in a range from 50 to 500 μm.     -   C) a bulk density in a range from 0.5 to 1.2 g/cm³, for example         in a range from 0.6 to 1.1 g/cm³, particularly preferably in a         range from 0.7 to 1.0 g/cm³;     -   D) a carbon content of less than 50 ppm;     -   E) an aluminium content of less than 200 ppb;     -   F) a tamped density in a range from 0.7 to 1.2 g/cm³;     -   G) a pore volume in a range from 0.1 to 2.5 mL/g, for example in         a range from 0.15 to 1.5 mL/g; particularly preferably in a         range from 0.2 to 0.8 mL/g;     -   H) an angle of repose in a range from 23 to 26°;     -   I) a particle size distribution D₁₀ in a range from 50 to 150         μm;     -   J) a particle size distribution D₅₀ in a range from 150 to 300         μm;     -   K) a particle size distribution D₉₀ in a range from 250 to 620         μm,     -   wherein the ppm and ppb are each based on the total weight of         the silicon dioxide granulate.

Preferably, the granules of the silicon dioxide granulate have a spherical morphology. Spherical morphology means a round or oval form of the particle. The granules of the silicon dioxide granulate preferably have a mean sphericity in a range from 0.7 to 1.3 SPHT3, for example a mean sphericity in a range from 0.8 to 1.2 SPHT3, particularly preferably a mean sphericity in a range from 0.85 to 1.1 SPHT3. The feature SPHT3 is described in the test methods.

Furthermore, the granules of the silicon dioxide granulate preferably have a mean symmetry in a range from 0.7 to 1.3 Symm3, for example a mean symmetry in a range from 0.8 to 1.2 Symm3, particularly preferably a mean symmetry in a range from 0.85 to 1.1 Symm3. The feature of the mean symmetry Symm3 is described in the test methods.

Preferably, the silicon dioxide granulate has a metal content of metals different to aluminium of less than 1000 ppb, for example of less than 500 ppb, particularly preferably of less than 100 ppb, in each case based on the total weight of the silicon dioxide granulate. Often however, the silicon dioxide granulate has a content of metals different to aluminium of at least 1 ppb. Often, the silicon dioxide granulate has a metal content of metals different to aluminium of less than 1 ppm, preferably in a range from 40 to 900 ppb, for example in a range from 50 to 700 ppb, particularly preferably in a range from 60 to 500 ppb, in each case based on the total weight of the silicon dioxide granulate. Such metals are for example sodium, lithium, potassium, magnesium, calcium, strontium, germanium, copper, molybdenum, titanium, iron and chromium. These can for example be present as an element, as an ion, or as part of a molecule or of an ion or of a complex.

The silicon dioxide granulate can comprise further constituents, for example in the form of molecules, ions or elements. Preferably, the silicon dioxide granulate comprises less than 500 ppm of further constituents, for example less than 300 ppm, particularly preferably less than 100 ppm, in each case based on the total weight of the silicon dioxide granulate. Often, at least 1 ppb of further constituents are comprised. The further constituents can in particular be selected from the group consisting of carbon, fluoride, iodide, bromide, phosphorus or a mixture of at least two thereof.

Preferably, the silicon dioxide granulate comprises less than 10 ppm carbon, for example less than 8 ppm or less than 5 ppm, particularly preferably less than 4 ppm, in each case based on the total weight of the silicon dioxide granulate. Often, at least 1 ppb of carbon is comprised in the silicon dioxide granulate.

Preferably, the silicon dioxide granulate comprises less than 100 ppm of further constituents, for example less than 80 ppm, particularly preferably less than 70 ppm, in each case based on the total weight of the silicon dioxide granulate. Often however, at least 1 ppb of the further constituents are comprised in the silicon dioxide granulate.

Preferably, step II. comprises the following steps:

-   -   II.1. Providing a liquid;     -   II.2. Mixing the silicon dioxide powder with the liquid to         obtain a slurry comprising silicon dioxide powder;     -   II.3. Granulating the slurry comprising silicon dioxide powder.

In the context of the present invention, a liquid means a material or a mixture of materials which is liquid at a pressure of 1013 hPa and a temperature of 20° C.

A “slurry” in the context of the present invention means a mixture of at least two materials, wherein the mixture, considered under the prevailing conditions, comprises at least one liquid and at least one solid.

Suitable liquids are all materials and mixtures of materials known to the skilled man and which appear suitable for the present application. Preferably, the liquid is selected from the group consisting of organic liquids and water. Preferably, the solubility of the silicon dioxide powder in the liquid is less than 0.5 g/L, preferably less than 0.25 g/L, particularly preferably less than 0.1 g/L, the g/L each given as g silicon dioxide powder per litre liquid.

Preferred suitable liquids are polar solvents. These can be organic liquids or water. Preferably, the liquid is selected from the group consisting of water, methanol, ethanol, n-propanol, iso-propanol, n-butanol, tert-butanol and mixtures of more than one thereof. Particularly preferably, the liquid is water. Particularly preferably, the liquid comprises distilled or de-ionised water.

Preferably, the silicon dioxide powder is processed to obtain a slurry comprising silicon dioxide powder. The silicon dioxide powder is virtually insoluble in the liquid at room temperature, but can be introduced into the liquid in high weight proportions to obtain the slurry comprising silicon dioxide powder.

The silicon dioxide powder and the liquid can be mixed in any manner. For example, the silicon dioxide powder can be added to the liquid, or the liquid can be added to the silicon dioxide powder. The mixture can be agitated during the addition or following the addition. Particularly preferably, the mixture is agitated during and following the addition. Examples for the agitation are shaking and stirring, or a combination of both. Preferably, the silicon dioxide powder can be added to the liquid under stirring. Furthermore, preferably, a portion of the silicon dioxide powder can be added to the liquid, wherein the mixture thus obtained is agitated, and the mixture is subsequently mixed with the remaining portion of the silicon dioxide powder. Likewise, a portion of the liquid can be added to the silicon dioxide powder, wherein the mixture thus obtained is agitated, and the mixture subsequently mixed with the remaining portion of the liquid.

By mixing the silicon dioxide powder and the liquid, a slurry comprising silicon dioxide powder is obtained. Preferably, the slurry comprising silicon dioxide powder is a suspension in which the silicon dioxide powder is distributed uniformly in the liquid. “Uniform” means that the density and the composition of the slurry comprising silicon dioxide powder at each position does not deviate from the average density and from the average composition by more than 10%, in each case based on the total amount of slurry comprising silicon dioxide powder. A uniform distribution of the silicon dioxide powder in the liquid can prepared, or obtained, or both, by an agitation as mentioned above.

Preferably, the slurry comprising silicon dioxide powder has a weight per litre in the range from 1000 to 2000 g/L, for example in the range from 1200 to 1900 g/L or from 1300 to 1800 g/L, particularly preferably in the range from 1400 to 1700 g/L. The weight per litre is measured by weighing a volume calibrated container

According to a preferred embodiment, at least one, for example at least two or at least three or at least four, particularly preferably at least five of the following features applies to the slurry comprising silicon dioxide powder:

-   -   a.) the slurry comprising silicon dioxide powder is transported         in contact with a plastic surface;     -   b.) the slurry comprising silicon dioxide powder is sheared;     -   c.) the slurry comprising silicon dioxide powder has a         temperature of more than 0° C., preferably in a range from 5 to         35° C.;     -   d.) the slurry comprising silicon dioxide powder has a zeta         potential at a pH value of 7 in a range from 0 to −100 mA, for         example from −20 to −60 mA, particularly preferably from −30 to         −45 mA;     -   e.) the slurry comprising silicon dioxide powder has a pH value         in a range of 7 or more, for example of more than 7 or a pH         value in the range from 7.5 to 13 or from 8 to 11, particularly         preferably from 8.5 to 10;     -   f.) the slurry comprising silicon dioxide powder has an         isoelectric point of less than 7, for example in a range from 1         to 5 or in a range from 2 to 4, particularly preferably in a         range from 3 to 3.5;     -   g.) the slurry comprising silicon dioxide powder has a solids         content of at least 40 wt.-%, for example in a range from 50 to         80 wt.-%, or in a range from 55 to 75 wt.-%, particularly         preferably in a range from 60 to 70 wt.-%, in each case based on         the total weight of the slurry;     -   h.) the slurry comprising silicon dioxide powder has a viscosity         according to DIN 53019-1 (5 rpm, 30 wt.-%) in a range from 500         to 1000 mPas, for example in the range from 600 to 900 mPas or         from 650 to 850 mPas, particularly preferably in the range from         700 to 800 mPas;     -   i.) the slurry comprising silicon dioxide powder has a         thixotropy according to DIN SPEC 91143-2 (30 wt.-% in water, 23°         C., 5 rpm/50 rpm) in the range from 3 to 6, for example in the         range from 3.5 to 5, particularly preferably in the range from         4.0 to 4.5;     -   j.) the silicon dioxide particles in the slurry comprising         silicon dioxide powder have in a 4 wt.-% slurry comprising         silicon dioxide powder a mean particle size in suspension         according to DIN ISO 13320-1 in the range from 100 to 500 nm,         for example in a range from 200 to 300 nm.

Preferably, the silicon dioxide particles in a 4 wt.-% aqueous slurry comprising silicon dioxide powder have a particle size D₁₀ in a range from 50 to 250 nm, particularly preferably in the range from 100 to 150 mm Preferably, the silicon dioxide particles in a 4 wt.-% aqueous slurry comprising silicon dioxide powder have a particle size D₅₀ in a range from 100 to 400 nm, particularly preferably in the range from 200 to 250 nm. Preferably, the silicon dioxide particles in a 4 wt.-% aqueous slurry comprising silicon dioxide powder have a particle size D₉₀ in a range from 200 to 600 nm, particularly preferably in a range from 350 to 400 nm. The particle size is measured according to DIN ISO 13320-1.

“Isoelectric point” means the pH value at which the zeta potential takes the value 0. The zeta potential is measured according to ISO 13099-2:2012.

Preferably, the pH value of the slurry comprising silicon dioxide powder is set to a value in the range given above. Preferably, the pH value can be set by adding to the slurry comprising silicon dioxide powder materials such as NaOH or NH₃, for example as aqueous solution. During this process, the slurry comprising silicon dioxide powder is often agitated.

Granulation

The silicon dioxide granulate is obtained from the silicon dioxide powder by granulation. Granulation means the transformation of powder particles into granules. During granulation, larger agglomerates which are referred to as “silicon dioxide granules” are formed by agglomeration of multiple silicon dioxide powder particles. These are often also called “silicon dioxide particles”, “silicon dioxide granulate particles” or “granulate particles”. Collectively, granules make up a granulate, e.g. the silicon dioxide granules make up a “silicon dioxide granulate”.

In the present case, any granulation process which is known to the skilled man and appears to him to be suitable for the granulation of silicon dioxide powder can in principle be selected. Granulation processes can be categorised as agglomeration granulation processes or press granulation processes, and further categorised as wet and dry granulation processes. Known methods are roll granulation in a granulation plate, spray granulation, centrifugal pulverisation, fluidised bed granulation, granulation processes employing a granulation mill, compactification, roll pressing, briquetting, scabbing or extruding.

Spray Drying

According to a preferred embodiment of the first aspect of the invention, a silicon dioxide granulate is obtained by spray granulation of the slurry comprising silicon dioxide powder. Spray granulation is also known as spray drying.

Spray drying is preferably effected in a spray tower. For spray drying, the slurry comprising silicon dioxide powder is preferably put under pressure at a raised temperature. The pressurised slurry comprising silicon dioxide powder is then depressurised via a nozzle and thus sprayed into the spray tower. Subsequently, droplets form which instantly dry and first form dry minute particles (“nuclei”). The minute particles form, together with a gas flow applied to the particles, a fluidised bed. In this way, they are maintained in a floating state and can thus form a surface for drying further droplets.

The nozzle, through which the slurry comprising silicon dioxide powder is sprayed into the spray tower, preferably forms an inlet into the interior of the spray tower.

The nozzle preferably has a contact surface with the slurry comprising silicon dioxide powder during spraying. “Contact surface” means the region of the nozzle which comes into contact with the slurry comprising silicon dioxide powder during spraying. Often, at least part of the nozzle is formed as a tube through which the slurry comprising silicon dioxide powder is guided during spraying, so that the inner side of the hollow tube comes into contact with the slurry comprising silicon dioxide powder.

The contact surface preferably comprises a glass, a plastic or a combination thereof. Preferably, the contact surface comprises a glass, particularly preferably quartz glass. Preferably, the contact surface comprises a plastic. In principle, all plastics known to the skilled man, which are stable at the process temperatures and do not pass any foreign atoms to the slurry comprising silicon dioxide powder, are suitable. Preferred plastics are polyolefins, for example homo- or co-polymers comprising at least one olefin, particularly preferably homo- or co-polymers comprising polypropylene, polyethylene, polybutadiene or combinations of two or more thereof. Preferably, the contact surface is made of a glass, a plastic or a combination thereof, for example selected from the group consisting of quartz glass and polyolefins, particularly preferably selected from the group consisting of quartz glass and homo- or co-polymers comprising polypropylene, polyethylene, polybutadiene or combinations of two or more thereof. Preferably, the contact surface comprises no metals, in particular no tungsten, titanium, tantalum, chromium, cobalt, nickel, iron, vanadium, zirconium and manganese

It is in principle possible for the contact surface and the further parts of the nozzle to be made of the same or from different materials. Preferably, the further parts of the nozzle comprise the same material as the contact surface. It is likewise possible for the further parts of the nozzle to comprise a material different to the contact surface. For example, the contact surface can be coated with a suitable material, for example with a glass or with a plastic.

Preferably, the nozzle is more than 70 wt.-%, based on the total weight of the nozzle, made out of an item selected from the group consisting of glass, plastic or a combination of glass and plastic, for example more than 75 wt.-% or more than 80 wt.-% or more than 85 wt.-% or more than 90 wt.-% or more than 95 wt.-%, particularly preferably more than 99 wt.-%.

Preferably, the nozzle comprises a nozzle plate. The nozzle plate is preferably made of glass, plastic or a combination of glass and plastic. Preferably, the nozzle plate is made of glass, particularly preferably quartz glass. Preferably, the nozzle plate is made of plastic. Preferred plastics are polyolefins, for example homo- or co-polymers comprising at least one olefin, particularly preferably homo- or co-polymers comprising polypropylene, polyethylene, polybutadiene or combinations of two or more thereof. Preferably, the nozzle plate comprises no metals, in particular no tungsten, titanium, tantalum, chromium, cobalt, nickel, iron, vanadium, zirconium and manganese.

Preferably, the nozzle comprises a screw twister. The screw twister is preferably made of glass, plastic or a combination of glass and plastic. Preferably, the screw twister is made of glass, particularly preferably quartz glass. Preferably, the screw twister is made of plastic. Preferred plastics are polyolefins, for example homo- or co-polymers comprising at least one olefin, particularly preferably homo- or co-polymers comprising polypropylene, polyethylene, polybutadiene or combinations of two or more thereof. Preferably, the screw twister comprises no metals, in particular no tungsten, titanium, tantalum, chromium, cobalt, nickel, iron, vanadium, zirconium and manganese

Furthermore, the nozzle can comprise further constituents. Preferred further constituents are a nozzle body, particularly preferable is a nozzle body which surrounds the screw twister and the nozzle plate, a cross piece and a baffle. Preferably, the nozzle comprises one or more, particularly preferably all, of the further constituents. The further constituents can independently from each other be made of in principle any material which is known to the skilled man and which is suitable for this purpose, for example of a metal comprising material, of glass or of a plastic. Preferably, the nozzle body is made of glass, particularly preferably quartz glass. Preferably, the further constituents are made of plastic. Preferred plastics are polyolefins, for example homo- or co-polymers comprising at least one olefin, particularly preferably homo- or co-polymers comprising polypropylene, polyethylene, polybutadiene or combinations of two or more thereof. Preferably, the further constituents comprise no metals, in particular no tungsten, titanium, tantalum, chromium, cobalt, nickel, iron, vanadium, zirconium and manganese

Preferably, the spray tower comprises a gas inlet and a gas outlet. Through the gas inlet, a gas can be introduced into the interior of the spray tower, and through the gas outlet it can be let out. It is also possible to introduce gas into the spray tower via the nozzle. Likewise, gas can be let out via the outlet of the spray tower. Furthermore, gas can preferably be introduced via the nozzle and a gas inlet of the spray tower, and let out via the outlet of the spray tower and a gas outlet of the spray tower.

Preferably, in the interior of the spray tower is present an atmosphere selected from air, an inert gas, at least two inert gases or a combination of air with at least one inert gas, preferably a combination of air with at least one inert gas, and preferably two inert gases. Inert gasses are preferably selected from the list consisting of nitrogen, helium, neon, argon, krypton and xenon. For example, in the interior of the spray tower there is present air, nitrogen or Argon, particularly preferably air.

Further preferably, the atmosphere present in the spray tower is part of a gas flow. The gas flow is preferably introduced into the spray tower via a gas inlet and let out via a gas outlet. It is also possible to introduce parts of the gas flow via the nozzle and to let out parts of the gas flow via a solids outlet. The gas flow can take on further constituents in the spray tower. These can come from the slurry comprising silicon dioxide powder during the spray drying and transfer to the gas flow.

Preferably, a dry gas flow is fed to the spray tower. A dry gas flow means a gas or a gas mixture which has a relative humidity at the temperature set in the spray tower below the condensation point. A relative air humidity of 100% corresponds to a water content of 17.5 g/m³ at 20° C. The gas is preferably pre-warmed to a temperature in a range from 150 to 450° C., for example from 200 to 420° C. or from 300 to 400° C., particularly preferably from 350 to 400° C.

The interior of the spray tower is preferably temperature-controllable. Preferably, the temperature in the interior of the spray tower has a value up to 550° C., for example 300 to 500° C., particularly preferably 350 to 450° C.

The gas flow preferably has a temperature at the gas inlet in a range from 150 to 450° C., for example from 200 to 420° C. or from 300 to 400° C., particularly preferably from 350 to 400° C.

The gas flow which is let out at the solids outlet, at the gas outlet or at both locations, preferably has a temperature of less than 170° C., for example from 50 to 150° C., particularly preferably from 100 to 130° C.

Furthermore, the difference between the temperature of the gas flow on introduction and of the gas flow on expulsion is preferably in a range from 100 to 330° C., for example from 150 to 300° C.

The thus obtained silicon dioxide granules are present as an agglomerate of individual particles of silicon dioxide powder. The individual particles of the silicon dioxide powder continue to be recognizable in the agglomerate. The mean particle size of the particles of the silicon dioxide powder is preferably in the range from 10 to 1000 nm, for example in the range from 20 to 500 nm or from 30 to 250 nm or from 35 to 200 nm or from 40 to 150 nm, or particularly preferably in the range from 50 to 100 nm. The mean particle size of these particles is measured according to DIN ISO 13320-1.

The spray drying can be carried out in the presence of auxiliaries. In principle, all materials can be employed as auxiliaries, which are known to the skilled man and which appear suitable for the present application. As auxiliary material for example, so-called binders can be considered. Examples of suitable binding materials are metal oxides such as calcium oxide, metal carbonates such as calcium carbonate and polysaccharides such as cellulose, cellulose ether, starch and starch derivatives.

Particularly preferably, the spray drying is carried out in the context of the present invention without auxiliaries.

Preferably, before, after or before and after the removal of the silicon dioxide granulate from the spray tower a portion thereof is separated off. For separating off, all processes which are known to the skilled man and which appear suitable can be considered. Preferably, the separating off is effected by a screening or a sieving.

Preferably, before removal from the spray tower of the silicon dioxide granulate which have been formed by spray drying, particles with a particle size of less than 50 μm, for example with a particle size of less than 70 μm particularly preferably with a particle size of less than 90 μm are separated off by screening. The screening is effected preferably using a cyclone arrangement, which is preferably arranged in the lower region of the spray tower, particularly preferably above the outlet of the spray tower.

Preferably, after removal of the silicon dioxide granulate from the spray tower, particles with a particle size of greater than 1000 μm, for example with a particle size of greater than 700 μm, particularly preferably with a particle size of greater than 500 μm are separated off by sieving. The sieving of the particles can in principle be effected in accordance with all processes known to the skilled man and which are suitable for this purpose. Preferably, the sieving is effected using a vibrating chute.

According to a preferred embodiment, the spray drying of the slurry comprising silicon dioxide powder through a nozzle into a spray tower is characterised by at least one, for example two or three, particularly preferably all of the following features:

-   -   a] spray granulation in a spray tower;     -   b] the presence of a pressure of the slurry comprising silicon         dioxide powder at the nozzle of not more than 40 bar, for         example in a range from 1.3 to 20 bar, from 1.5 to 18 bar or         from 2 to 15 bar or from 4 to 13 bar, or particularly preferably         in the range from 5 to 12 bar, wherein the pressure is given in         absolute terms (relative to p=0 hPa);     -   c] a temperature of the droplets upon entering into the spray         tower in a range from 10 to 50° C., preferably in a range from         15 to 30° C., particularly preferably in a range from 18 to 25°         C.     -   d] a temperature at the side of the nozzle directed towards the         spray tower in a range from 100 to 450° C., for example in a         range from 250 to 440° C., particularly preferably from 350 to         430° C.;     -   e] A throughput of slurry comprising silicon dioxide powder         through the nozzle in a range from 0.05 to 1 m³/h, for example         in a range from 0.1 to 0.7 m³/h or from 0.2 to 0.5 m³/h,         particularly preferably in a range from 0.25 to 0.4 m³/h;     -   f] A solids content of the slurry comprising silicon dioxide         powder of at least 40 wt.-%, for example in a range from 50 to         80 wt.-%, or in a range from 55 to 75 wt.-%, particularly         preferably in a range from 60 to 70 wt.-%, in each case based on         the total weight of the slurry comprising silicon dioxide         powder;     -   g] A gas inflow into the spray tower in a range from 10 to 100         kg/min, for example in a range from 20 to 80 kg/min or from 30         to 70 kg/min, particularly preferably in a range from 40 to 60         kg/min;     -   h] A temperature of the gas flow upon entering into the spray         tower in a range from 100 to 450° C., for example in a range         from 250 to 440° C., particularly preferably from 350 to 430°         C.;     -   i] A temperature of the gas flow at the exit out of the spray         tower of less than 170° C.;     -   j] The gas is selected from the group consisting of air,         nitrogen and helium, or a combination of two or more thereof;         preferably air;     -   k] a residual moisture content of the granulate on removal out         of the spray tower of less than 5 wt.-%, for example of less         than 3 wt.-% or of less than 1 wt.-% or in a range from 0.01 to         0.5 wt.-%, particularly preferably in a range from 0.1 to 0.3         wt.-%, in each case based on the total weight of the silicon         dioxide granulate created in the spray drying;     -   l] at least 50 wt.-% of the spray granulate, based on the total         weight of the silicon dioxide granulate created in the spray         drying, completes a flight time in a range from 1 to 100 s, for         example of a period from 10 to 80 s, particularly preferably         over a period from 25 to 70 s;     -   m] at least 50 wt.-% of the spray granulate, based on the total         weight of the silicon dioxide granulate created in the spray         drying, covers a flight path of more than 20 m, for example of         more than 30 or of more than 50 or of more than 70 or of more         than 100 or of more than 150 or of more than 200 or in a range         from 20 to 200 m or from 10 to 150 or from 20 to 100,         particularly preferably a range from 30 to 80 m.     -   n] the spray tower has a cylindrical geometry;     -   o] a height of the spray tower of more than 10 m, for example of         more than 15 m or of more than 20 m or of more than 25 m or of         more than 30 m or in a range from 10 to 25 m, particularly         preferably in a range from 15 to 20 m;     -   p] screening out of particles with a size of less than 90 μm         before the removal of the granulate from the spray tower;     -   q] sieving out of particles with a size of more than 500 μm         after the removal of the granulate from the spray tower,         preferably in a vibrating chute;     -   r] The exit of the droplets of the slurry comprising silicon         dioxide powder out of the nozzle occurs at an angle of 30 to 60         degrees from vertical, particularly preferably at an angle of 45         degree from vertical.

Vertical means the direction of the gravitational force vector.

The flight path means the path covered by a droplet of slurry comprising silicon dioxide powder from exiting out of the nozzle in the gas chamber of the spray tower to form a granule up to completion of the action of flying and falling. The action of flying and falling frequently ends by the granule impacting with the floor of the spray tower impacting or the granule impacting with other granules already lying on the floor of the spray tower, whichever occurs first.

The flight time is the period required by a granule to cover the flight path in the spray tower. Preferably, the granules have a helical flight path in the spray tower.

Preferably, at least 60 wt.-% of the spray granulate, based on the total weight of the silicon dioxide granulate created in the spray drying, cover a mean flight path of more than 20 m, for example of more than 30 or of more than 50 or of more than 70 or of more than 100 or of more than 150 or of more than 200 or in a range from 20 to 200 m or from 10 to 150 or from 20 to 100, particularly preferably a range from 30 to 80 m.

Preferably, at least 70 wt.-% of the spray granulate, based on the total weight of the silicon dioxide granulate created in the spray drying, cover a mean flight path of more than 20 m, for example of more than 30 or of more than 50 or of more than 70 or of more than 100 or of more than 150 or of more than 200 or in a range from 20 to 200 m or from 10 to 150 or from 20 to 100, particularly preferably a range from 30 to 80 m.

Preferably, at least 80 wt.-% of the spray granulate, based on the total weight of the silicon dioxide granulate created in the spray drying, cover a mean flight path of more than 20 m, for example of more than 30 or of more than 50 or of more than 70 or of more than 100 or of more than 150 or of more than 200 or in a range from 20 to 200 m or from 10 to 150 or from 20 to 100, particularly preferably a range from 30 to 80 m.

Preferably, at least 90 wt.-% of the spray granulate, based on the total weight of the silicon dioxide granulate created in the spray drying, cover a mean flight path of more than 20 m, for example of more than 30 or of more than 50 or of more than 70 or of more than 100 or of more than 150 or of more than 200 or in a range from 20 to 200 m or from 10 to 150 or from 20 to 100, particularly preferably a range from 30 to 80 m.

Roll Granulation

According to a preferred embodiment of the first aspect of the invention of the invention, a silicon dioxide granulate is obtained by roll granulation of the slurry comprising silicon dioxide powder.

The roll granulation is carried out by stirring the slurry comprising silicon dioxide powder in the presence of a gas at raised temperature. Preferably, the roll granulation is effected in a stirring vessel fitted with a stirring tool. Preferably, the stirring vessel rotates in the opposite sense to the stirring tool. Preferably, the stirring vessel additionally comprises an inlet through which the silicon dioxide powder can be introduced into the stirring vessel, an outlet through which the silicon dioxide granulate can be removed, a gas inlet and a gas outlet.

For stirring the slurry comprising silicon dioxide powder, preferably a pin-type stirring tool is used. A pin-type stirring tool means a stirring tool fitted with multiple elongate pins having their longitudinal axis coaxial with the rotational axis of the stirring tool. The trajectory of the pins preferably traces coaxial circles around the axis of rotation.

Preferably, the slurry comprising silicon dioxide powder is set to a pH value of less than 7, for example to a pH value in the range from 2 to 6.5, particularly preferably to a pH value in a range from 4 to 6. For setting the pH value, an inorganic acid is preferably used, for example an acid selected from the group consisting of hydrochloric acid, sulphuric acid, nitric acid and phosphoric acid, particularly preferably hydrochloric acid.

Preferably, in the stirring vessel is present an atmosphere selected from air, an inert gas, at least two inert gases or a combination of air with at least one inert gas, preferably at least two inert gases. Inert gases are preferably selected from the list consisting of nitrogen, helium, neon, argon, krypton and xenon. For example, air, nitrogen or argon is present in the stirring vessel, particularly preferably air.

Furthermore, preferably, the atmosphere present in the stirring vessel is part of a gas flow. The gas flow is preferably introduced into the stirring vessel via the gas inlet and let out via the gas outlet. The gas flow can take on further constituents in the stirring vessel. These can originate from the slurry comprising silicon dioxide powder in the roll granulation and transfer into the gas flow.

Preferably, a dry gas flow is introduced to the stirring vessel. A dry gas flow means a gas or a gas mixture which has a relative humidity at the temperature set in the stirring vessel under the condensation point. The gas is preferably pre-warmed to a temperature in a range from 50 to 300° C., for example from 80 to 250° C., particularly preferably from 100 to 200° C.

Preferably, per 1 kg of the employed slurry comprising silicon dioxide powder, 10 to 150 m³ gas per hour is introduced into the stirring vessel, for example 20 to 100 m³ gas per hour, particularly preferably 30 to 70 m³ gas per hour.

During the mixing, the slurry comprising silicon dioxide powder is dried by the gas flow to form silicon dioxide granules. The granulate which is formed is removed from the stirring vessel.

Preferably, the removed granulate is dried further. Preferably, the drying is effected continuously, for example in a rotary kiln. Preferred temperatures for the drying are in a range from 80 to 250° C., for example in a range from 100 to 200° C., particularly preferably in a range from 120 to 180° C.

In the context of the present invention, continuous in respect of a process means that it can be operated continuously. That means that the introduction and removal of materials involved in the process can be effected on an ongoing basis whilst the process is being run. It is not necessary to interrupt the process for this.

Continuous as an attribute of an object, e.g. in relation to a “continuous oven”, means that this object is configured in such a way that a process carried out therein, or a process step carried out therein, can be carried out continuously.

The granulate obtained from the roll granulation can be sieved. The sieving occur before or after the drying. Preferably it is sieved before drying. Preferably, granule with a particle size of less than 50 μm, for example with a particle size of less than 80 μm, particularly preferably with a particle size of less than 100 μm, are sieved out. Furthermore, preferably, granules with a particle size of greater than 900 μm, for example with a particle size of greater than 700 μm, particularly preferably with a particle size of greater than 500 μm, sieved out. The sieving out of larger particles can in principle be carried out in accordance with any process known to the skilled man and which is suitable for this purpose. Preferably, the sieving out of larger particles is carried out by means of a vibrating chute.

According to a preferred embodiment, the roll granulation is characterised by at least one, for example two or three, particularly preferably all of the following features:

-   -   [a] The granulation is carried out in a rotating stirring         vessel;     -   [b] The granulation is carried out in a gas flow of 10 to 150 kg         gas per hour and per 1 kg slurry comprising silicon dioxide         powder;     -   [c] The gas temperature on introduction is 40 to 200° C.;     -   [d] Granules with a particle size of less than 100 μm and of         more than 500 μm are sieved out;     -   [e] The granules formed have a residual moisture content of 15         to 30 wt.-%;     -   [f] The granules formed are dried at 80 to 250° C., preferably         in a continuous drying tube, particularly preferably to a         residual moisture content of less than 1 wt.-%.

Preferably, the silicon dioxide granules obtained by granulation, preferably by spray- or roll-granulation, also referred to as silicon dioxide granulate I, is treated before it is processed to obtain quartz glass bodies. This pretreatment can fulfil various purposes which either facilitate the processing to obtain quartz glass bodies or influence the properties of the resulting quartz glass body. For example, the silicon dioxide granulate I can be compactified, purified, surface-modified or dried.

Preferably, the silicon dioxide granulate I can by subjected to a thermal, mechanical or chemical treatment or a combination of two or more treatments, wherein a silicon dioxide granulate II is obtained.

Chemical

According to a preferred embodiment of the first aspect of the invention, the silicon dioxide granulate I has a carbon content w_(C(1)). The carbon content w_(C(1)) is preferably less than 50 ppm, for example less than 40 ppm or less than 30 ppm, particularly preferably in a range from 1 ppb to 20 ppm, each based on the total weight of the silicon dioxide granulate I.

According to a preferred embodiment of the first aspect of the invention, the silicon dioxide granulate I comprises at least two particles. Preferably, the at least two particles can carry out a motion relative to each other. As means for bringing about the relative motion, in principle all means known to the skilled man and which appear to him to be suitable can be considered. Particular preferred is a mixing. A mixing can in principle be carried out in any manner. Preferably, a feed-oven is selected for this. Accordingly, the at least two particles can preferably perform a motion relative to each other by being agitated in a feed oven, for example in a rotary kiln.

Feed ovens mean ovens for which loading and unloading of the oven, so-called charging, is carried out continuously. Examples of feed-ovens are rotary kilns, roll-over type furnaces, belt conveyor ovens, conveyor ovens, continuous pusher-type furnaces. Preferably, for treatment of the silicon dioxide granulate I, rotary kilns are used.

According to a preferred embodiment of the first aspect of the invention, the silicon dioxide granulate I is treated with a reactant to obtain a silicon dioxide granulate II. The treatment is carried out in order to change the concentration of certain materials in the silicon dioxide granulate. The silicon dioxide granulate I can have impurities or certain functionalities, the content of which should be reduced, such as for example: OH groups, carbon containing compounds, transition metals, alkali metals and alkali earth metals. The impurities and functionalities can originate from the starting materials or can be introduced in the course of the process. The treatment of the silicon dioxide granulate I can serve various purposes. For example, employing treated silicon dioxide granulate I, i.e. silicon dioxide granulate II, can simplify the processing of the silicon dioxide granulate to obtain quartz glass bodies. Furthermore, this selection can be employed to tune the properties of the resulting quartz glass body. For example, the silicon dioxide granulate I can be purified or surface modified. The treatment of the silicon dioxide granulate I can thus be employed for improving the properties of the resulting quartz glass bodies.

Preferably, a gas or a combination of multiple gases is suitable as reactant. This is also referred to as a gas miture. In principle, all gases known to the skilled man can be employed, which are known for the specified treatment and which appear to be suitable. Preferably, a gas selected from the group consisting of HCl, Cl₂, F₂, O₂, O₃, H₂, C₂F₄, C₂F₆, HClO₄, air, inert gas, e.g. N₂, He, Ne, Ar, Kr, or combinations of two or more thereof is employed. Preferably, the treatment is carried out in the presence of a gas or a combination of two or more gases.

Preferably, the treatment is carried out in a gas counter flow or a gas co-flow.

Preferably, the reactant is selected from the group consisting of HCl, Cl₂, F₂, O₂, O₃ or combinations of two or more thereof. Preferably, mixtures of two or more of the above-mentioned gases are used for the treatment of silicon dioxide granulate I. Through the presence of F, Cl or both, metals which are contained in silicon dioxide granulates I as impurities, such as for example transition metals, alkali metals and alkali earth metals, can be removed. In this connection, the above mentioned metals can be converted along with constituents of the gas mixture under the process conditions to obtain gaseous compounds which are subsequently drawn out and thus are no longer present in the granulate. Furthermore, preferably, the OH content in the silicon dioxide granulate I can be decreased by the treatment of the silicon dioxide granulate I with these gases.

Preferably, a gas mixture of HCl and Cl₂ is employed as reactant. Preferably, the gas mixture has an HCl content in a range from 1 to 30 vol.-%, for example in a range from 2 to 15 vol.-%, particularly preferably in a range from 3 to 10 vol.-%. Likewise, the gas mixture preferably has a Cl₂ content in a range from 20 to 70 vol.-%, for example in a range from 25 to 65 vol.-%, particularly preferably in a range from 30 to 60 vol.-%. The remainder up to 100 vol.-% can be made up of one or more inert gases, e.g. N₂, He, Ne, Ar, Kr, or of air. Preferably, the proportion of inert gas in the reactants is in a range from 0 to less than 50 vol.-%, for example in a range from 1 to 40 vol.-% or from 5 to 30 vol.-%, particularly preferably in a range from 10 to 20 vol.-%, in each case based on the total volume of the reactants.

O₂, C₂F₂, or mixtures thereof with Cl₂ are preferably used for purifying silicon dioxide granulate I which has been prepared from a siloxane or from a mixture of multiple siloxanes.

The reactant in the form of a gas or of a gas mixture is preferably contacted with the silicon dioxide granulate as a gas flow or as part of a gas flow with a throughput in a range from 50 to 2000 L/h, for example in a range from 100 to 1000 L/h, particularly preferably in a range from 200 to 500 L/h. A preferred embodiment of the contacting is a contact of the gas flow and silicon dioxide granulate in a feed oven, for example in a rotary kiln. Another preferred embodiment of the contacting is a fluidised bed process.

Through treatment of the silicon dioxide granulate I with the reactant, a silicon dioxide granulate II with a carbon content w_(C(2)) is obtained. The carbon content w_(C(2)) of the silicon dioxide granulate II is less than the carbon content w_(C(1)) of the silicon dioxide granulate I, based on the total weight of the respective silicon dioxide granulate. Preferably, w_(C(2)) is 0.5 to 99%, for example 20 to 80% or 50 to 95%, particularly preferably 60 to 99% less than w_(C(1)).

Thermal

Preferably, the silicon dioxide granulate I is additionally subjected to a thermal or mechanical treatment or to a combination of these treatments. One or more of these additional treatments can be carried out before or during the treatment with the reactant. Alternatively, or additionally, the additional treatment can also be carried out on the silicon dioxide granulate II. In what follows, the term “silicon dioxide granulate” comprises the alternatives “silicon dioxide granulate I” and “silicon dioxide granulate II”. It is equally possible to carry out the treatments described in the following to the “silicon dioxide granulate I”, or to the treated silicon dioxide granulate I, the “silicon dioxide granulate II”.

The treatment of the silicon dioxide granulate can serve various purposes. For example, this treatment facilitates the processing of the silicon dioxide granulate to obtain quartz glass bodies. The treatment can also influence the properties of the resulting glass body. For example, the silicon dioxide granulate can be compactified, purified, surface modified or dried. In this connection, the specific surface area (BET) can decrease. Likewise, the bulk density and the mean particle size can increase due to agglomerations of silicon dioxide particles. The thermal treatment can be carried out dynamically or statically.

For the dynamic thermal treatment, all ovens in which the silicon dioxide granulate can be thermally treated whilst being agitated are in principle suitable. For the dynamic thermal treatment, preferably feed ovens are used.

A preferred mean holding time of the silicon dioxide granulate in the dynamic thermal treatment is quantity dependent. Preferably, the mean holding time of the silicon dioxide granulate in the dynamic thermal treatment is in the range from 10 to 180 min, for example in the range from 20 to 120 min or from 30 to 90 min. Particularly preferably, the mean holding time of the silicon dioxide granulate in the dynamic thermal treatment is in the range from 30 to 90 min.

In the case of a continuous process, a defined portion of the flow of silicon dioxide granulate is used as a sample load for the measurement of the holding time, e.g. a gram, a kilogram or a tonne. The start and end of the holding time are determined by the introduction into and exiting from the continuous oven operation.

Preferably, the throughput of the silicon dioxide granulate in a continuous process for dynamic thermal treatment is in the range from 1 to 50 kg/h, for example in the range from 5 to 40 kg/h or from 8 to 30 kg/h. Particularly preferably, the throughput here is in the range from 10 to 20 kg/h.

In the case of a discontinuous process for dynamic thermal treatment, the treatment time is given as the period of time between the loading and subsequent unloading of the oven.

In the case of a discontinuous process for dynamic thermal treatment, the throughput is in a range from 1 to 50 kg/h, for example in the range from 5 to 40 kg/h or from 8 to 30 kg/h. Particularly preferably, the throughput is in the range from 10 to 20 kg/h. The throughput can be achieved using a sample load of a determined amount which is treated for an hour. According to another embodiment, the throughput can be achieved through a number of loads per hour, wherein the weight of a single load corresponds to the throughput per hour divided by the number of loads. In this event, time of treatment corresponds to the fraction of an hour which is given by 60 minutes divided by the number of loads per hour.

Preferably, the dynamic thermal treatment of the silicon dioxide granulate is carried out at an oven temperature of at least 500° C., for example in the range from 510 to 1700° C. or from 550 to 1500° C. or from 580 to 1300° C., particularly preferably in the range from 600 to 1200° C.

Normally, the oven has the indicated temperature in the oven chamber. Preferably, this temperature deviates from the indicated temperature by less than 10% downwards or upwards, based on the entire treatment period and the entire length of the oven as well as at every point in time in the treatment as well as at every position in the oven.

Alternatively, in particular the continuous process of a dynamic thermal treatment of the silicon dioxide granulate can by carried out at differing oven temperatures. For example, the oven can have a constant temperature over the treatment period, wherein the temperature varies in section over the length of the oven. Such sections can be of the same length or of different lengths. Preferably, in this case, the temperature increases from the entrance of the oven to the exit of the oven. Preferably, the temperature at the entrance is at least 100° C. lower than at the exit, for example 150° C. lower or 200° C. lower or 300° C. lower or 400° C. lower. Furthermore, preferably, the temperature at the entrance is preferably at least 500° C., for example in the range from 510 to 1700° C. or from 550 to 1500° C. or from 580 to 1300° C., particularly preferably in the range from 600 to 1200° C. Furthermore, preferably, the temperature at the entrance is preferably at least 300° C., for example from 400 to 1000° C. or from 450 to 900° C. or from 500 to 800° C. or from 550 to 750° C., particularly preferably from 600 to 700° C. Furthermore, each of the temperature ranges given at the oven entrance can be combined with each of the temperature ranges given at the oven exit. Preferred combinations of oven entrance temperature ranges and oven exit temperature ranges are:

Oven entrance temperature range Oven exit temperature range [° C.] [° C.]  400-1000 510-1300 450-900 550-1260 480-850 580-1200 500-800 600-1100 530-750 630-1050

For the static thermal treatment of the silicon dioxide granulate crucibles arranged in an oven are preferably used. Suitable crucibles are sinter crucibles or metal sheet crucibles. Preferred are rolled metal sheet crucibles made out of multiple sheets which are riveted together. Examples of crucible materials are refractory metals, in particular tungsten, molybdenum and tantalum. The crucible can furthermore be made of graphite or in the case of the crucible of refractory metals can be lined with graphite foil. Furthermore, preferably, the crucibles can be made of silicon dioxide. Particularly preferably, silicon dioxide crucibles are employed.

The mean holding time of the silicon dioxide granulate in the static thermal treatment is quantity dependent. Preferably, the mean holding time of the silicon dioxide granulate in the static thermal treatment for a 20 kg amount of silicon dioxide granulate I is in the range from 10 to 180 min, for example in the range from 20 to 120 min, particularly preferably in the range from 30 to 90 min.

Preferably, the static thermal treatment of the silicon dioxide granulate is carried out at an oven temperature of at least 800° C., for example in the range from 900 to 1700° C. or from 950 to 1600° C. or from 1000 to 1500° C. or from 1050 to 1400° C., particularly preferably in the range from 1100 to 1300° C.

Preferably, the static thermal treatment of the silicon dioxide granulate I is carried out at constant oven temperature. The static thermal treatment can also be carried out at a varying oven temperature. Preferably, in this case, the temperature increases during the treatment, wherein the temperature at the start of the treatment is at least 50° C. lower than at the end, for example 70° C. lower or 80° C. lower or 100° C. lower or 110° C. lower, and wherein the temperature at the end is preferably at least 800° C., for example in the range from 900 to 1700° C. or from 950 to 1600° C. or from 1000 to 1500° C. or from 1050 to 1400° C., particularly preferably in the range from 1100 to 1300° C.

Mechanical

According to a further preferred embodiment, the silicon dioxide granulate I can be mechanically treated. The mechanical treatment can be carried out for increasing the bulk density. The mechanical treatment can be combined with the above mentioned thermal treatment. A mechanical treatment can avoid the agglomerates in the silicon dioxide granulate and therefore the mean particle size of the individual, treated silicon dioxide granules in the silicon dioxide granulate becoming too large. An enlargement of the agglomerates can hinder the further processing or have disadvantageous impacts on the properties of the quartz glass bodies prepared by the inventive process, or a combination of both effects. A mechanical treatment of the silicon dioxide granulate also promotes a uniform contact of the surfaces of the individual silicon dioxide granules with the gas or gases. This is in particular achieved by concurrent mechanical and chemical treatment with one or more gases. In this way, the effect of the chemical treatment can be improved.

The mechanical treatment of the silicon dioxide granulate can be carried out by moving two or more silicon dioxide granules relative to each other, for example by rotating the tube of a rotary kiln.

Preferably, the silicon dioxide granulate I is treated chemically, thermally and mechanically. Preferably, a simultaneous chemical, thermal and mechanical treatment of the silicon dioxide granulate I is carried out.

In the chemical treatment, the content of impurities in the silicon dioxide granulate I is reduced. For this, the silicon dioxide granulate I can be treated in a rotary kiln at raised temperature and under a chlorine and oxygen containing atmosphere. Water present in the silicon dioxide granulate I evaporates, organic materials react to form CO and CO₂. Metal impurities can be converted to volatile chlorine containing compounds.

Preferably, the silicon dioxide granulate I is treated in a chlorine and oxygen containing atmosphere in a rotary kiln at a temperature of at least 500° C., preferably in a temperature range from 550 to 1300° C. or from 600 to 1260° C. or from 650 to 1200° C. or from 700 to 1000° C., particularly preferably in a temperature range from 700 to 900° C. The chlorine containing atmosphere contains for example HCl or Cl₂ or a combination of both. This treatment causes a reduction of the carbon content.

Furthermore, preferably alkali and iron impurities are reduced. Preferably, a reduction of the number of OH groups is achieved. At temperatures under 700° C., treatment periods can be long, at temperatures above 1100° C. there is a risk that pores of the granulate close, trapping chlorine or gaseous chlorine compounds.

Preferably, it is also possible to carry out sequentially multiple chemical treatment steps, each concurrent with thermal and mechanical treatment. For example, the silicon dioxide granulate I can first be treated in a chlorine containing atmosphere and subsequently in an oxygen containing atmosphere. The low concentrations of carbon, hydroxyl groups and chlorine resulting therefrom facilitate the melting down of the silicon dioxide granulate II.

According to a further preferred embodiment, step 11.2) is characterised by at least one, for example by at least two or at least three, particularly preferably by a combination of all of the following features:

-   -   N1) The reactant comprises HCl, Cl₂ or a combination therefrom;     -   N2) The treatment is carried out in a rotary kiln;     -   N3) The treatment is carried out at a temperature in a range         from 600 to 900° C.;     -   N4) The reactant forms a counter flow;     -   N5) The reactant has a gas flow in a range from 50 to 2000 L/h,         preferably 100 to 1000 L/h, particularly preferably 200 to 500         L/h;     -   N6) The reactant has a volume proportion of inert gas in a range         from 0 to less than 50 vol.-%.

Preferably, the silicon dioxide granulate I has a particle diameter which is greater than the particle diameter of the silicon dioxide powder. Preferably, the particle diameter of the silicon dioxide granulate I is up to 300 times as great as the particle diameter of the silicon dioxide powder, for example up to 250 times as great or up to 200 times as great or up to 150 times as great or up to 100 times as great or up to 50 times as great or up to 20 times as great or up to 10 times as great, particularly preferably 2 to 5 times as great.

The silicon dioxide granulate obtained in this way is also called silicon dioxide granulate II. Particularly preferably, the silicon dioxide granulate II is obtained from the silicon dioxide granulate I in a rotary kiln by means of a combination of thermal, mechanical and chemical treatment.

The silicon dioxide granulate provided in step i.) is preferably selected from the group consisting of silicon dioxide granulate I, silicon dioxide granulate II and a combination therefrom.

“Silicon dioxide granulate I” means a granulate of silicon dioxide which is produced by granulation of silicon dioxide powder which was obtained through pyrolysis of silicon compounds in a fuel gas flame. Preferred fuel gases are oxyhydrogen gas, natural gas or methane gas, particularly preferable is oxyhydrogen gas.

“Silicon dioxide granulate II” means a granulate of silicon dioxide which is produced by post treatment of the silicon dioxide granulate I. Possible post treatments are chemical, thermal and/or mechanical treatments. This is described at length in the context of the description of the provision of the silicon dioxide granulate (process step II. of the first aspect of the invention).

Particularly preferably, the silicon dioxide granulate provided in step i.) is the silicon dioxide granulate I. The silicon dioxide granulate I has the following features:

-   -   [A] a BET surface area in the range from 20 to 50 m²/g, for         example in a range from 20 to 40 m²/g; particularly preferably         in a range from 25 to 35 m²/g; wherein the micro pore portion         preferably accounts for a BET surface area in a range from 4 to         5 m²/g; for example in a range from 4.1 to 4.9 m²/g;         particularly preferably in a range from 4.2 to 4.8 m²/g; and     -   [B] a mean particle size in a range from 180 to 300 μm.

Preferably, the silicon dioxide granulate I is characterised by at least one, for example by at least two or at least three or at least four, particularly preferably by at least five of the following features:

-   -   [C] a bulk density in a range from 0.5 to 1.2 g/cm³, for example         in a range from 0.6 to 1.1 g/cm³, particularly preferably in a         range from 0.7 to 1.0 g/cm³;     -   [D] a carbon content of less than 50 ppm, for example less than         40 ppm or less than 30 ppm or less than 20 ppm or less than 10         ppm, particularly preferably in a range from 1 ppb to 5 ppm;     -   [E] an aluminium content of less than 200 ppb, preferably of         less than 100 ppb, for example of less than 50 ppb or from 1 to         200 ppb or from 15 to 100 ppb, particularly preferably in a         range from 1 to 50 ppb.     -   [F] a tamped density in a range from 0.5 to 1.2 g/cm³, for         example in a range from 0.6 to 1.1 g/cm³, particularly         preferably in a range from 0.75 to 1.0 g/cm³;     -   [G] a pore volume in a range from 0.1 to 1.5 mL/g, for example         in a range from 0.15 to 1.1 mL/g; particularly preferably in a         range from 0.2 to 0.8 mL/g,     -   [H] a chlorine content of less than 200 ppm, preferably of less         than 150 ppm, for example less than 100 ppm, or of less than 50         ppm, or of less than 1 ppm, or of less than 500 ppb, or of less         than 200 ppb, or in a range from 1 ppb to less than 200 ppm, or         from 1 ppb to 100 ppm, or from 1 ppb to 1 ppm, or from 10 ppb to         500 ppb, or from 10 ppb to 200 ppb, particularly preferably from         1 ppb to 80 ppb;     -   [I] metal content of metals which are different to aluminium of         less than 1000 ppb, preferably in a range from 1 to 900 ppb, for         example in a range from 1 to 700 ppb, particularly preferably in         a range from 1 to 500 ppb;     -   [J] a residual moisture content of less than 10 wt.-%,         preferably in a range from 0.01 wt.-% to 5 wt.-%, for example         from 0.02 to 1 wt.-%, particularly preferably from 0.03 to 0.5         wt.-%;     -   wherein the wt.-%, ppm and ppb are each based on the total         weight of the silicon dioxide granulate I.

The OH content, or hydroxyl group content, means the content of OH groups in a material, for example in silicon dioxide powder, in silicon dioxide granulate or in a quartz glass body. The content of OH groups is measured spectroscopically in the infrared by comparing the first and the third OH bands.

The chlorine content means the content of elemental chlorine or chlorine ions in the silicon dioxide granulate, in the silicon dioxide powder or in the quartz glass body.

The aluminium content means the content of elemental aluminium or aluminium ions in the silicon dioxide granulate, in the silicon dioxide powder or in the quartz glass body.

Preferably, the silicon dioxide granulate I has a micro pore proportion in a range from 4 to 5 m²/g; for example in a range from 4.1 to 4.9 m²/g; particularly preferably in a range from 4.2 to 4.8 m²/g.

The silicon dioxide granulate I preferably has a density in a range from 2.1 to 2.3 g/cm³, particularly preferably in a range from 2.18 to 2.22 g/cm³.

The silicon dioxide granulate I preferably has a mean particle size in a range from 180 to 300 μm, for example in a range from 220 to 280 μm, particularly preferably in a range from 230 to 270 μm.

The silicon dioxide granulate I preferably has a particle size D₅₀ in a range from 150 to 300 μm, for example in a range from 180 to 280 μm, particularly preferably in a range from 220 to 270 μm. Furthermore, preferably, the silicon dioxide granulate I has a particle size D₁₀ in a range from 50 to 150 μm, for example in a range from 80 to 150 μm, particularly preferably in a range from 100 to 150 μm. Furthermore, preferably, the silicon dioxide granulate I has a particle size D₉₀ in a range from 250 to 620 μm, for example in a range from 280 to 550 μm, particularly preferably in a range from 300 to 450 μm.

The silicon dioxide granulate I preferably has the feature combination [A]/[B]/[C] or [A]/[B]/[E] or [A]/[B]/[G], further preferred the feature combination [A]/[B]/[C]/[E] or [A]/[B]/[C]/[G] or [A]/[B]/[E]/[G], further preferably the feature combination [A]/[B]/[C]/[E]/[G].

The silicon dioxide granulate I preferably has the feature combination [A]/[B]/[C], wherein the BET surface area is in a range from 20 to 40 m²/g, the mean particle size is in a range from 180 to 300 μm and the bulk density is in a range from 0.6 to 1.1 g/mL.

The silicon dioxide granulate I preferably has the feature combination [A]/[B]/[E], wherein the BET surface area is in a range from 20 to 40 m²/g, the mean particle size is in a range from 180 to 300 μm and the aluminium content is in a range from 1 to 50 ppb.

The silicon dioxide granulate I preferably has the feature combination[A]/[B]/[G], wherein the BET surface area is in a range from 20 to 40 m²/g, the mean particle size is in a range from 180 to 300 μm and the pore volume is in a range from 0.2 to 0.8 mL/g.

The silicon dioxide granulate I preferably has the feature combination [A]/[B]/[C]/[E], wherein the BET surface area is in a range from 20 to 40 m²/g, the mean particle size is in a range from 180 to 300 μm, the bulk density is in a range from 0.6 to 1.1 g/mL and the aluminium content is in a range from 1 to 50 ppb.

The silicon dioxide granulate I preferably has the feature combination [A]/[B]/[C]/[G], wherein the BET surface area is in a range from 20 to 40 m²/g, the mean particle size is in a range from 180 to 300 μm, the bulk density is in a range from 0.6 to 1.1 g/mL and the pore volume is in a range from 0.2 to 0.8 mL/g.

The silicon dioxide granulate I preferably has the feature combination [A]/[B]/[E]/[G], wherein the BET surface area is in a range from 20 to 40 m²/g, the mean particle size is in a range from 180 to 300 μm, the aluminium content is in a range from 1 to 50 ppb and the pore volume is in a range from 0.2 to 0.8 mL/g.

The silicon dioxide granulate I preferably has the feature combination [A]/[B]/[C]/[E]/[G], wherein the BET surface area is in a range from 20 to 40 m²/g, the mean particle size is in a range from 180 to 300 μm, the bulk density is in a range from 0.6 to 1.1 g/mL, the aluminium content is in a range from 1 to 50 ppb and the pore volume is in a range from 0.2 to 0.8 mL/g.

Particle size means the size of the particles of aggregated primary particles, which are present in a silicon dioxide powder, in a slurry or in a silicon dioxide granulate. The mean particle size means the arithmetic mean of all particle sizes of the indicated material. The D₅₀ value indicates that 50% of the particles, based on the total number of particles, are smaller than the indicated value. The D₁₀ value indicates that 10% of the particles, based on the total number of particles, are smaller than the indicated value. The D₉₀ value indicates that 90% of the particles, based on the total number of particles, are smaller than the indicated value. The particle size is measured by the dynamic photo analysis process according to ISO 13322-2:2006-11.

Furthermore, particularly preferably, the silicon dioxide granulate provided in step i.) is the silicon dioxide granulate II. The silicon dioxide granulate II has the following features:

-   -   (A) a BET surface area in the range from 10 to 35 m²/g, for         example in a range from 10 to 30 m²/g,particularly preferably in         a range from 20 to 30 m²/g; and     -   (B) a mean particle size in a range from 100 to 300 μm, for         example in a range from 150 to 280 μm or from 200 to 270 μm,         particularly preferably in a range from 230 to 260 μm.

Preferably, the silicon dioxide granulate II has at least one, for example at least two or at least three or at least four, particularly preferably at least five of the following features:

-   -   (C) a bulk density in a range from 0.7 to 1.2 g/cm³, for example         in a range from 0.75 to 1.1 g/cm³, particularly preferably in a         range from 0.8 to 1.0 g/cm³;     -   (D) a carbon content of less than 5 ppm, for example less than         4.5 ppm or in a range from 1 ppb to 4 ppm, particularly         preferably of less than 4 ppm;     -   (E) an aluminium content of less than 200 ppb, for example of         less than 150 ppb or of less than 100 ppb or from 1 to 150 ppb         or from 1 to 100 ppb, particularly preferably in a range from 1         to 80 ppb;     -   (F) a tamped density in a range from 0.7 to 1.2 g/cm³, for         example in a range from 0.75 to 1.1 g/cm³, particularly         preferably in a range from 0.8 to 1.0 g/cm³;     -   (G) a pore volume in a range from 0.1 to 2.5 mL/g, for example         in a range from 0.2 to 1.5 mL/g; particularly preferably in a         range from 0.4 to 1 mL/g;     -   (H) a chlorine content of less than 500 ppm, preferably of less         than 400 ppm, for example less than 350 ppm or preferably of         less than 330 ppm or in a range from 1 ppb to 500 ppm or from 10         ppb to 450 ppm particularly preferably from 50 ppb to 300 ppm;     -   (I) a metal content of metals which are different to aluminium         of less than 1000 ppb, for example in a range from 1 to 400 ppb,         particularly preferably in a range from 1 to 200 ppb;     -   (J) a residual moisture content of less than 3 wt.-%, for         example in a range from 0.001 wt.-% to 2 wt.-%, particularly         preferably from 0.01 to 1 wt.-%,     -   wherein the wt.-%, ppm and ppb are each based on the total         weight of the silicon dioxide granulate II.

Preferably, the silicon dioxide granulate II has a micro pore proportion in a range from 1 to 2 m²/g, for example in a range from 1.2 to 1.9 m²/g, particularly preferably in a range from 1.3 to 1.8 m²/g.

The silicon dioxide granulate II preferably has a density in a range from 0.5 to 2.0 g/cm³, for example from 0.6 to 1.5 g/cm³, particularly preferably from 0.8 to 1.2 g/cm³. The density is measured according to the method described in the test methods.

The silicon dioxide granulate II preferably has a particle size D₅₀ in a range from 150 to 250 μm, for example in a range from 180 to 250 μm, particularly preferably in a range from 200 to 250 μm. Furthermore, preferably, the silicon dioxide granulate II has a particle size D₁₀ in a range from 50 to 150 μm, for example in a range from 80 to 150 μm, particularly preferably in a range from 100 to 150 μm. Furthermore, preferably, the silicon dioxide granulate II has a particle size D₉₀ in a range from 250 to 450 μm, for example in a range from 280 to 420 μm, particularly preferably in a range from 300 to 400 μm.

The silicon dioxide granulate II preferably has the feature combination (A)/(B)/(D) or (A)/(B)/(F) or (A)/(B)/(I), further preferred the feature combination (A)/(B)/(D)/(F) or (A)/(B)/(D)/(I) or (A)/(B)/(F)/(I), further preferably the feature combination (A)/(B)/(D)/(F)/(I).

The silicon dioxide granulate II preferably has the feature combination (A)/(B)/(D), wherein the BET surface area is in a range from 10 to 30 m²/g, the mean particle size is in a range from 150 to 280 μm and the carbon content is less than 4 ppm.

The silicon dioxide granulate II preferably has the feature combination (A)/(B)/(F), wherein the BET surface area is in a range from 10 to 30 m²/g, the mean particle size is in a range from 150 to 280 μm and the tamped density is in a range from 0.8 to 1.0 g/mL.

The silicon dioxide granulate II preferably has the feature combination (A)/(B)/(I), wherein the BET surface area is in a range from 10 to 30 m²/g, the mean particle size is in a range from 150 to 280 μm and the metal content of metals which are different to aluminium is in a range from 1 to 400 ppb.

The silicon dioxide granulate II preferably has the feature combination (A)/(B)/(D)/(F), wherein the BET surface area is in a range from 10 to 30 m²/g, the mean particle size is in a range from 150 to 280 μm, the carbon content is less than 4 ppm and the tamped density is in a range from 0.8 to 1.0 g/mL.

The silicon dioxide granulate II preferably has the feature combination (A)/(B)/(D)/(I), wherein the BET surface area is in a range from 10 to 30 m²/g, the mean particle size is in a range from 150 to 280 μm, the carbon content is less than 4 ppm and the metal content of metals which are different to aluminium is in a range from 1 to 400 ppb.

The silicon dioxide granulate II preferably has the feature combination (A)/(B)/(F)/(I), wherein the BET surface area is in a range from 10 to 30 m²/g, the mean particle size is in a range from 150 to 280 μm, the tamped density is in a range from 0.8 to 1.0 g/mL and the metal content of metals which are different to aluminium is in a range from 1 to 400 ppb.

The silicon dioxide granulate II preferably has the feature combination (A)/(B)/(D)/(F)/(I), wherein the BET surface area is in a range from 10 to 30 m²/g, the mean particle size is in a range from 150 to 280 μm, the carbon content is less than 4 ppm, the tamped density is in a range from 0.8 to 1.0 g/mL and the metal content of metals which are different to aluminium is in a range from 1 to 400 ppb.

Step ii.)

According to step ii.) , a glass melt is formed out of the silicon dioxide granulate. Normally, the silicon dioxide granulate is warmed until a glass melt is obtained. The warming of the silicon dioxide granulate to obtain a glass melt can in principle by carried out by any way known to the skilled man for this purpose.

Preparation of a Glass Melt

The formation of a glass melt from the silicon dioxide granulate, for example by warming, can be carried out by a continuous process. In the process according to the invention for the preparation of a quartz glass body, the silicon dioxide granulate can preferably be introduced continuously into an oven or the glass melt can be removed continuously from the oven, or both. Particularly preferably, the silicon dioxide granulate is introduced continuously into the oven and the glass melt is removed continuously from the oven.

For this, an oven which has at least one inlet and at least one outlet is in principle suitable. An inlet means an opening through which silicon dioxide and optionally further materials can be introduced into the oven. An outlet means an opening through which at least a part of the silicon dioxide can be removed from the oven. The oven can for example be arranged vertically or horizontally. Preferably, the oven is arranged vertically. Preferably, at least one inlet is located above at least one outlet. “Above” in connection with fixtures and features of an oven means, in particular in connection with an inlet and outlet, that the fixture or the features which is arranged “above” another has a higher position above the zero of absolute height. “Vertical” means that the line directly joining the inlet and the outlet of the oven deviates not more than 30° from the direction of gravity.

According to a preferred embodiment of the first aspect of the invention, the oven comprises a hanging metal sheet crucible. Into the hanging metal sheet crucible is introduced the silicon dioxide granulate and warmed to obtain a glass melt. A metal sheet crucible means a crucible which comprises at least one rolled metal sheet. Preferably, a metal sheet crucible has multiple rolled metal sheets. A hanging metal sheet crucible means a metal sheet crucible as previously described which is arranged in an oven in a hanging position.

The hanging metal sheet crucible can in principle be made of all materials which are known to the skilled man and which are suitable for melting silicon dioxide. Preferably, the metal sheet of the hanging metal sheet crucible comprises a sintered material, for example a sinter metal. Sinter metals means metals or alloys which are obtained by sintering of metal powders.

Preferably, the metal sheet of the metal sheet crucible comprises at least one item selected from the group consisting of the refractory metals. Refractory metals means metals of group 4 (Ti, Zr, Hf), of group 5 (V, Nb, Ta) and of group 6 (Cr, Mo, W).

Preferably, the metal sheet of the metal sheet crucible comprises a sinter metal selected from the group consisting of molybdenum, tungsten or a combination thereof. Furthermore, preferably, the metal sheet of the metal sheet crucible comprises at least one further refractory metal, particularly preferably rhenium, osmium, iridium, ruthenium or a combination of two or more thereof.

Preferably, the metal sheet of the metal sheet crucible comprises an alloy of molybdenum with a refractory metal, or tungsten with a refractory metal. Particularly preferred alloy metals are rhenium, osmium, iridium, ruthenium or a combination of two or more thereof. According to a further example, the metal sheet of the metal sheet crucible is an alloy of molybdenum with tungsten, rhenium, osmium, iridium, ruthenium or a combination of two or more thereof. For example the metal sheet of the metal sheet crucible can be an alloy of tungsten with molybdenum, rhenium, osmium, iridium, ruthenium or a combination of two or more thereof.

Preferably, the above described metal sheet of the metal sheet crucible can be coated with a refractory metal. According to a preferred example, the metal sheet of the metal sheet crucible is coated with rhenium, osmium, iridium, ruthenium, molybdenum or tungsten, or a combination of two or more thereof.

Preferably, the metal sheet and the coating have different compositions. For example a molybdenum metal sheet can be coated with one or multiple coats of rhenium, osmium, iridium, ruthenium, tungsten or a combination of two or more thereof. According to another example, a tungsten metal sheet is coated with one or multiple layers of rhenium, osmium, iridium, ruthenium, molybdenum or a combination of two or more thereof. According to a further example, the metal sheet of the metal sheet crucible can be made of molybdenum alloyed with rhenium or of tungsten alloyed with rhenium, and be coated on the inner side of the crucible with one or multiple layers comprising rhenium, osmium, iridium, ruthenium or a combination of two or more thereof.

Preferably, the metal sheet of the hanging metal sheet crucible has a density 95% or greater of the theoretical density, for example from 95% to 98% or from 96% to 98%. More preferable are higher theoretical densities, in particular in the range from 98 to 99.95%. The theoretical density of a basic material corresponds to the density of a pore free and 100% dense material. A density of the metal sheet of the metal sheet crucible of more than 95% of the theoretical density can for example be obtained by sintering a sinter metal and subsequent compactification of the sintered metal. Particularly preferably, a metal sheet crucible is obtainable by sintering of a sinter metal, rolling to obtain a metal sheet and processing the metal sheet to obtain a crucible.

Preferably, the metal sheet crucible has at least a lid, a wall and a base plate. Preferably, the hanging metal sheet crucible has at least one, for example at least two or at least three or at least four, particularly preferably at least five or all of the following features:

-   -   (a) at least one, e.g. more than one or at least two or at least         three or at least five, particularly preferably three or four         layers of the metal sheet;     -   (b) at least one metal sheet, e.g. at least three or at least         four or at least six or at least eight or at least twelve or at         least 15 or at least 16 or at least 20 metal sheets,         particularly preferably twelve or 16 metal sheets;     -   (c) at least one join between two metal sheet parts, e.g. at         least two or at least five or at least ten or at least 18 or at         least 24 or at least 36 or at least 48 or at least 60 or at         least 72 or at least 48 or at least 96 or at least 120 or at         least 160, particularly preferably 36 or 48 joins between two of         the same or between multiple different metal sheet parts of the         hanging metal sheet crucible;     -   (d) The metal sheet parts of the hanging metal sheet crucible         are riveted, e.g. by deep drawing at least one join e.g. joined         by a combination of deep drawing with metal sheet collaring or         countersinking, screwed or welded e.g. electron beam welding and         sintering of the weld points, particularly preferably riveted;     -   (e) The metal sheet of the hanging metal sheet crucible is         obtainable by a shaping step which is associated with an         increase of the physical density, preferably by shaping of a         sintered metal or of a sintered alloy; furthermore, preferably,         the shaping is a rolling;     -   (f) A hanger assembly of copper, aluminium, steel, iron, nickel         or a refractory metal, e.g. of the crucible material, preferably         a water cooled hanger assembly of copper or steel;     -   (g) A nozzle, preferably a nozzle permanently fixed to the         crucible;     -   (h) A mandrel, for example a mandrel fixed to the nozzle with         pins or a mandrel fixed to the lid with a supporting rod or a         mandrel attached underneath the crucible with a supporting rod;     -   (i) at least one gas inlet, e.g. in the form of a filling pipe         or as a separate inlet;     -   (j) at least one gas outlet, e.g. as a separate outlet in the         lid or in the wall of the crucible;     -   (k) a cooled jacket, preferably a water cooled jacket;     -   (l) an insulation on the outside, preferably an insulation on         the outside made of zirconium oxide.

The hanging metal sheet crucible can in principle be heated in any way which is known to the skilled person and which seems to him to be suitable. The hanging metal sheet crucible can for example be heated by means of electrical heating elements (resistive) or by induction. In the case of resistive heating, the solid surface of the metal sheet crucible is warmed from the outside and delivers the energy from there to its inner side.

According to a preferred embodiment of the present invention, the energy transfer into the melting crucible is not performed by warming the melting crucible, or a bulk material present therein, or both, using a flame, such as for example a burner flame directed into the melting crucible or onto the melting crucible.

By means of the hanging arrangement, the hanging metal sheet crucible can be moved in the oven. Preferably, the crucible can be at least partially moved into and moved out of the oven. If different heating zones are present in the oven, their temperature profile will be transferred to the crucible which is present in the oven. By changing the position of the crucible in the oven, multiple heating zones, varying heating zones or multiple varying heating zones can be produced in the crucible.

The metal sheet crucible has a nozzle. The nozzle is made of a nozzle material. Preferably, the nozzle material comprises a pre-compactified material, for example with a density in a range of more than 95%, for example from 98 to 100%, particularly preferably from 99 to 99.999%, in each case based on the theoretical density of the nozzle material. Preferably, the nozzle material comprises a refractory metal, for example molybdenum, tungsten or a combination thereof with a further refractory metal. Molybdenum is a particularly preferred nozzle material. Preferably, a nozzle comprising molybdenum can have a density of 100% of the theoretical density.

Preferably, the base plate comprised in a metal sheet crucible is thicker than the sides of the metal sheet crucible. Preferably, the base plate is made of the same material as the sides of the metal sheet crucible. Preferably, the base plate of the metal sheet crucible is not a rolled metal sheet. The base plate is for example 1.1 to 5000 times as thick or 2 to 1000 times as thick or 4 to 500 times as thick, particularly preferably 5 to 50 times as thick, each time compared with a wall of the metal sheet crucible.

According to a preferred embodiment of the first aspect of the invention, the oven comprises a hanging or a standing sinter crucible. The silicon dioxide granulate is introduced into the hanging or standing sinter crucible and warmed to obtain a glass melt.

A sinter crucible means a crucible which is made from a sinter material which comprises at least one sinter metal and has a density of not more than 96% of the theoretical density of the metal. Sinter metal means metals of alloys which are obtained by sintering of metal powders. The sinter material and the sinter metal in a sinter crucible are not rolled.

Preferably, the sinter material of the sinter crucible has a density of 85% or more of the theoretical density of the sinter material, for example a density from 85% to 95% or from 90% to 94%, particularly preferably from 91% to 93%.

The sinter material can in principle be made of any material which is known to the skilled person and which is suitable for melting silicon dioxide. Preferably, the sinter material is made of at least one of the elements selected from the group consisting of refractory metals, graphite or materials lined with graphite foil.

Preferably, the sinter material comprises a first sinter metal selected from the group consisting of molybdenum, tungsten and a combination thereof. Furthermore, preferably, the sinter material additionally comprises at least one further refractory metal which is different to the first sinter metal particularly preferably selected from the group consisting of molybdenum, tungsten, rhenium, osmium, iridium, ruthenium or a combination of two or more thereof.

Preferably, the sinter material comprises an alloy of molybdenum with a refractory metal, or tungsten with a refractory metal. Particularly preferable alloy metals are rhenium, osmium, iridium, ruthenium or a combination of two or more thereof. According to a further example, the sinter material comprises an alloy of molybdenum with tungsten, rhenium, osmium, iridium, ruthenium or a combination of two or more thereof. For example the sinter material can comprise an alloy of tungsten with molybdenum, rhenium, osmium, iridium, ruthenium or a combination of two or more thereof.

According to a further preferred embodiment, the above described sinter material can comprise a coating which comprises a refractory metal, in particular rhenium, osmium, iridium, ruthenium or a combination of two or more thereof. According to a preferred example, the coating comprises rhenium, osmium, iridium, ruthenium, molybdenum or tungsten, or a combination of two or more thereof.

Preferably, the sinter material and its coating have different compositions. An example is a sinter material comprising molybdenum which is coated with one or more layers of rhenium, osmium, iridium, ruthenium, tungsten or of a combination of two or more thereof. According to another example, a sinter material comprising tungsten is coated with one or more layers of rhenium, osmium, iridium, ruthenium, molybdenum or of a combination of two or more thereof. According to another example, the sinter material can be made of molybdenum alloyed with rhenium or of tungsten alloyed with rhenium, and coated on the inner side of the crucible with one or multiple layers comprising rhenium, osmium, iridium, ruthenium or comprising a combination of two or more thereof.

Preferably, a sinter crucible is made by sintering the sinter material to obtain a mould. The sinter crucible can be made in a mould as a whole. It is also possible for individual parts of the sinter crucible to be made in a mould and subsequently processed to obtain the sinter crucible. Preferably, the crucible is made out of more than one part, for example a base plate and one or more side parts. The side parts are preferably made in one piece, based on the circumference of the crucible. Preferably, the sinter crucible can be made of multiple side parts arranged on top of each other. Preferably, the side parts of the sinter crucible are sealed by means of screwing or by means of a tongue and groove connection. A screwing is preferably achieved by making side parts which have a thread at the borders. In the case of a tongue and groove connection, two side parts which are to be joined each have a notch at the borders into which tongue is introduced as the connecting third part, such that a form-closed connection is formed perpendicular to the plane of the crucible wall. Particularly preferably, a sinter crucible is made of more than one side part, for example of two or more side parts, particularly preferably of three or more side parts. Particularly preferably, the parts of the hanging sinter crucible are screwed. Particularly preferably, the parts of the standing sinter crucible are connected by means of a tongue and groove connection.

The base plate can in principle be connected with the crucible wall by any means known to the skilled person and which is suitable for this purpose. According to a preferred embodiment, the base plate has an outward thread and the base plate is connected with the crucible wall by being screwed into it. According to a further preferred embodiment, the base plate is connected with the crucible wall by means of screws. According to a further preferred embodiment, the base plate is suspended in the sinter crucible, for example by laying the base plate on an inward flange of the crucible wall. According to a further preferred embodiment, at least a part of the crucible wall and a compactified base plate are sintered in one piece. Particularly preferably, the base plate and the crucible wall of the hanging sinter crucible are screwed. Particularly preferably, the base plate and the crucible wall of the standing sinter crucible are connected by means of a tongue and groove connection.

Preferably, the base plate comprised by a sinter crucible is thicker than the sides, for example 1.1 to 20 times as thick or 1.2 to 10 times as thick or 1.5 to 7 times as thick, particularly preferably 2 to 5 times as thick. Preferably, the sides have a constant wall thickness over the circumference and over the height of the sinter crucible.

The sinter crucible has a nozzle. The nozzle is made of a nozzle material. Preferably, the nozzle material comprises a pre-compactified material, for example with a density in a range of more than 95%, for example from 98 to 100%, particularly preferably from 99 to 99.999%, in each case based on the theoretical density of the nozzle material. Preferably, the nozzle material comprises a refractory metal, for example molybdenum, tungsten or a combination therefrom with a refractory metal. Molybdenum is a particularly preferred nozzle material. Preferably, a nozzle comprising molybdenum can have a density of 100% of the theoretical density.

The hanging sinter crucible can be heated in any way known to the skilled person and which appears to him to be suitable. The hanging sinter crucible can for example be heated inductively or resistively. In the case of inductive heating, the energy is introduced directly via coils in the side wall of the sinter crucible and delivered from there to the inside of the crucible. In the case of resistive heating, the energy is introduced by radiation, whereby the solid surface is warmed from the outside and the energy is delivered from them to the inside. Preferably, the sinter crucible is heated inductively.

Preferably, the sinter crucible has one or more than one heating zones, for example one or two or three or more than three heating zones, preferably one or two or three heating zones, particularly preferably one heating zone. The heating zones of the sinter crucible can be brought up to the same temperature or different temperatures. For example, all heating zones can be brought up to one temperature, or all heating zones can be brought up to different temperatures, or two or more heating zones can be brought up to one temperature and one or more heating zones can, independently of each other, be brought up to other temperatures. Preferably, all heating zones are brought up to the different temperatures, for example the temperature of the heating zones increases in the direction of the material transport of the silicon dioxide granulate.

A hanging sinter crucible means a sinter crucible as previously described which is arranged hanging in an oven.

Preferably, the hanging sinter crucible has at least one, for example at least two or at least three or at least four, particularly preferably all of the following features:

-   -   {a} a hanging assembly, preferably a height adjustable hanging         assembly;     -   {b} at least two rings sealed together as side parts, preferably         at least two rings screwed to each other as side parts;     -   {c} a nozzle, preferably a nozzle which is permanently attached         to the crucible;     -   {d} a mandrel, for example a mandrel fixed to the nozzle with         pins or a mandrel fixed to the lid with a supporting rod or a         mandrel attached underneath the crucible with a supporting rod;     -   {e} at least one gas inlet, e.g. in the form of a filling pipe         or as a separate inlet, particularly preferably in the form of a         filling pipe;     -   {f} at least one gas outlet, e.g. at the lid or in the wall of         the crucible.     -   {g} A cooled jacket, particularly preferably a water cooled         jacket;     -   {h} An insulation on the outside of the crucible, for example on         the outside of the cooled jacket, preferably an insulation layer         made of zirconium oxide.

The hanging assembly is preferably a hanging assembly which is installed during the construction of the hanging sinter crucible, for example a hanging assembly which is provided as an integral component of the crucible, particularly preferably a hanging assembly which is provided out of the sinter material as an integral component of the crucible. Furthermore, the hanging assembly is preferably a hanging assembly which is installed onto the sinter crucible and which is made of a material which is different to the sinter material, for example of aluminium, steel, iron, nickel or copper, preferably of copper, particularly preferably a cooled, for example a water cooled, hanging assembly made of copper which is installed on the sinter crucible.

By virtue of the hanging assembly, the hanging sinter crucible can be moved in the oven. Preferably, the crucible can be at least partially introduced and withdrawn from the oven. If different heating zones are present in the oven, their temperature profile will be transferred to the crucible which is present in the oven. By changing the position of the crucible in the oven, multiple heating zones, varying heating zones or multiple varying heating zones can be produced in the crucible.

A standing sinter crucible means a sinter crucible of the type previously described which is arranged standing in an oven.

Preferably, the standing sinter crucible has at least one, for example at least two or at least three or at least four, particularly preferably all of the following features:

-   -   /a/A region formed as a standing area, preferably a region         formed as a standing area on the base of the crucible, further         preferably a region formed as a standing area in the base plate         of the crucible, particularly preferably a region formed as a         standing area at the outer edge of the base of the crucible;     -   /b/at least two rings sealed together as side parts, preferably         at least two rings sealed together by means of a tongue and         groove connection as side parts;     -   /c/a nozzle, preferably a nozzle which is permanently attached         to the crucible, particularly preferably a region of the base of         the crucible which is not formed as a standing area;     -   /d/a mandrel, for example a mandrel fixed to the nozzle with         pins or a mandrel fixed to the lid with pins or a mandrel         attached from underneath the crucible with supporting rod;     -   /e/at least one gas inlet, e.g. in the form of a filling tube or         as a separate inlet;     -   /f/at least one gas outlet, e.g. as a separate outlet in the lid         or in the wall of the crucible;     -   /g/a lid.

The standing sinter crucible preferably has a separation of the gas compartments in the oven and in the region underneath the oven. The region underneath the oven means the region underneath the nozzle, in which the removed glass melt is present. Preferably, the gas compartments are separated by the surface on which the crucible stands. Gas which is present in the gas compartment of the oven between the inner wall of the oven and the outer wall of the crucible, cannot leak down into the region underneath the oven. The removed glass melt does not contact the gases from the gas compartment of the oven. Preferably, glass melts removed from an oven with a sinter crucible in a standing arrangement and quartz glass bodies formed therefrom have a higher surface purity than melts removed from an oven with a sinter crucible in a hanging arrangement and quartz glass bodies formed therefrom.

Preferably, the crucible is connected with the inlet and the outlet of the oven in such a way that silicon dioxide granulate can enter into the crucible via the crucible inlet and through the inlet of the oven and glass melt can be removed through the outlet of the crucible and the outlet of the oven.

Preferably, the crucible comprises, in addition to the at least one inlet, at least one opening, preferably multiple openings, through which the gas can be introduced and removed. Preferably, the crucible comprises at least two openings, whereby at least one can be used as a gas inlet and at least one can be used as a gas outlet. Preferably, the use of at least one opening as gas inlet and at least one opening as gas outlet leads to a gas flow in the crucible.

The silicon dioxide granulate is introduced into the crucible through the inlet of the crucible and subsequently warmed in the crucible. The warming can be carried out in the presence of a gas or of a gas mixture of two or more gases. Furthermore, during the warming, water attached to the silicon dioxide granulate can transfer to the gas phase and form a further gas. The gas or the mixture of two or more gases is present in the gas compartment of the crucible. The gas compartment of the crucible means the region inside the crucible which is not occupied by a solid or liquid phase. Suitable gases are for example hydrogen, inert gases as well as two or more thereof. Inert gases mean those gases which up to a temperature of 2400° C. do not react with the materials present in the crucible. Preferred inert gases are nitrogen, helium, neon, argon, krypton and xenon, particularly preferably argon and helium. Preferably, the warming is carried out in reducing atmosphere. This can be provided by means of hydrogen or a combination of hydrogen and an inert gas, for example a combination of hydrogen and helium, or of hydrogen and nitrogen, or of hydrogen and argon, particularly preferably a combination of hydrogen and helium.

Preferably, an at least partial gas exchange of air, oxygen and water in exchange for hydrogen, at least one inert gas, or in exchange for a combination of hydrogen and at least one inert gas is carried out on the silicon dioxide granulate. The at least partial gas exchange is carried out on the silicon dioxide granulate during introduction of the silicon dioxide granulate, or before the warming, or during the warming, or during at least two of the aforementioned activities. Preferably, the silicon dioxide granulate is warmed to melting in a gas flow of hydrogen and at least one inert gas, for example argon or helium.

Preferably, the dew point of the gas on exiting through the gas outlet is less than 0° C. The dew point means the temperature beneath which at fixed pressure a part of the gas or gas mixture in question condenses. In general, this means the condensation of water. The dew point is measured with a dew point mirror hygrometer according to the test method described in the methods section.

Preferably, the oven has at least one gas outlet, as does preferably also a melting crucible found therein, through which gas introduced into the oven and gas formed during the running of the oven is removed. The oven can additionally have at least one dedicated gas inlet. Alternatively or additionally, gas can be introduced through the inlet, also referred to as the solids inlet, for example together with the silicon dioxide particles, or beforehand, afterwards, or by a combination of two or more of the aforementioned possibilities.

Preferably, the gas which is removed from the oven through the gas outlet has a dew point of less than 0° C., for example of less than −10° C., or less than −20° C. on exiting from the oven through the gas outlet. The dew point is measured according to the test method described in the methods section at a slight overpressure of 5 to 20 mbar. A suitable measuring device is for example an “Optidew” device from the company Michell Instruments GmbH, D-61381 Friedrichsdorf.

The dew point of the gas is preferably measured at a measuring location at a distance of 10 cm or more from the gas outlet of the oven. Often, this distance is between 10 cm and 5 m. In this range of distances—here referred to as “on exiting”—the distance of the measuring location from the gas outlet of the oven is insignificant for the result of the dew point measurement. The gas is conveyed to the measurement location by fluid connection, for example in a hose or a tube. The temperature of the gas at the measurement location is often between 10 and 60° C., for example 20 to 50° C., in particular 20 to 30° C.

Suitable gases and gas mixtures have already been described. It was established in the context of separate tests that the above disclosed values apply to each of the named gases and gas mixtures.

According to a further preferred embodiment, the gas or gas mixture has a dew point of less than −50° C. prior to entering into the oven, in particular into the melting crucible, for example less than −60° C., or less than −70° C., or less than −80° C. A dew point commonly does not exceed −60° C. Also, the following ranges for the dew point upon entering into the oven are preferred: from −50 to −100° C.; from −60 to −100° C. and from −70 to −100° C.

According to a further preferred embodiment, the dew point of the gas prior to entering into the oven is at least 50° C. less than on exiting from the melting crucible, for example at least 60° C., or even 80° C. For measuring the dew point on exiting from the melting crucible, the above disclosures apply. For measuring the dew point prior to entry into the oven, the disclosures apply analogously. Since no source of contribution to moisture is present and there is no possibility of condensing out between the measuring location and the oven, the distance of the measuring location to the gas inlet of the oven is not relevant.

According to a preferred embodiment, the oven, in particular the melting crucible, is operated with a gas exchange rate in a range from 200 to 3000 L/h.

According to a preferred embodiment, the dew point is measured in a measuring cell, the measuring cell being separated by a membrane from the gas passing through the gas outlet. The membrane is preferably permeable to moisture. By these means, the measuring cell can be protected from dust and other particles present in the gas flow and which are conveyed out of the melting oven, in particular out of a melting crucible, along with the gas flow By these means, the working time of a measuring probe can be increased considerably. The working time means the time period of operation of the oven during which neither replacement of the measuring probe, nor cleaning of the measuring probe is required.

According to a preferred embodiment, a dew point mirror measuring device is employed.

The dew point at the gas outlet of the oven can be configured. Preferably, a process for configuring the dew point at the outlet of the oven comprises the following steps:

-   -   I) Providing an input material in an oven, wherein the input         material has a residual moisture;     -   II) Operating the oven, wherein a gas flow is passed through the         oven, and     -   III) Varying the residual moisture of the input material, or the         gas replacement rate of the gas flow.

Preferably, this process can be used to configure the dew point to a range of less than 0° C., for example less than −10° C., particularly preferably less than −20° C. Further preferably, the dew point can be configured to a range of less than 0° C. to −100° C., for example less than −10° C. to −80° C., particularly preferably less than −20° C. to −60° C.

For the preparation of a quartz glass body, “Input material” means silicon dioxide particles which are provided, preferably silicon dioxide granulate, silicon dioxide grain, or combinations thereof. The silicon dioxide particles, the granulate and the grain are preferably characterised by the features described in the context of the first aspect.

The oven and the gas flow are preferably characterised by the features described in the context of the first aspect. Preferably, the gas flow is formed by introducing a gas into the oven through an inlet and by removing a gas out of the oven through an outlet. The “gas replacement rate” means the volume of gas which is passed out of the oven through the outlet per unit time. The gas replacement rate is also called the throughput of the gas flow or volume throughput.

The configuration of the dew point can in particular be performed by varying the residual moisture of the input material or the gas replacement rate of the gas flow. For example, the dew point can be increased by increasing residual moisture of the input material. By decreasing the residual moisture of the input material, the dew point can be reduced. An increase in the gas replacement rate can lead to a reduction in the dew point. A reduced gas replacement rate on the other hand can yield an increased dew point.

Preferably, the gas replacement rate of the gas flow is in a range from 200 to 3000 L/h, for example 200 to 2000 L/h, particularly preferably 200 to 1000 L/h.

The residual moisture of the input material is preferably in a range from 0.001 wt. % to 5 wt. %, for example from 0.01 to 1 wt. %, particularly preferably 0.03 to 0.5 wt. %, in each case based on the total weight of the input material.

Preferably, the dew point can also be affected by further factors. Examples of such means are the dew point of the gas flow on entry into the oven, the oven temperature and the composition of the gas flow. A reduction of the dew point of the gas flow on entry into the oven, a reduction of the oven temperature or a reduction of the temperature of the gas flow at the outlet of the oven can lead to a reduction of the dew point of the gas flow at the outlet. The temperature of the gas flow at the outlet of the oven has no effect on the dew point, as long as it is above the dew point.

Particularly preferably, the dew point is configured by varying the gas replacement rate of the gas flow.

Preferably, the process is characterised by at least one, for example at least two or at least three, particularly preferably at least four of the following feature:

-   -   I} A residual moisture of the input material in a range from         0.001 to 5 wt. %, for example from 0.01 to 1 wt. %, particularly         preferably from 0.03 to 0.5 wt. %, in each case based on the         total weight of the input material.     -   II} A gas replacement rate of the gas flow in a range from 200         to 3000 L/h, for example from 200 to 2000 L/h, particularly         preferably from 200 to 1000 L/h;     -   III} An oven temperature in a range from 1700 to 2500° C., for         example in a range from 1900 to 2400° C., particularly         preferably in a range from 2100 to 2300° C.;     -   IV} A dew point of the gas flow on entry into the oven in a         range from −50° C. to −100° C., for example from −60° C. to         −100° C., particularly preferably from −70° C. to −100° C.;     -   V} The gas flow comprises helium, hydrogen or a combination         thereof, preferably helium and hydrogen in a ratio from 20:80 to         95:5;     -   VI} A temperature of the gas at the outlet in a range from 10 to         60° C., for example from 20 to 50° C., particularly preferably         from 20 to 30° C.

It is particularly preferred, when employing a silicon dioxide granulate with a high residual moisture, to employ a gas flow with a high gas replacement rate and a low dew point at the inlet of the oven. By contrast, when employing a silicon dioxide granulate with a low residual moisture, a gas flow with a low gas replacement rate and a high dew point at the inlet of the oven can be used.

Particularly preferably, when employing a silicon dioxide granulate with a residual moisture of less than 3 wt. %, the gas replacement rate of a gas flow comprising helium and hydrogen can be in a range from 200 to 3000 L/h.

If a silicon dioxide granulate with a residual moisture of 0.1% is fed to the oven at 30 kg/h, a gas replacement rate of the gas flow in a range from 2800 to 3000 1/h is selected in the case of He/H₂=50:50 and in a range from 2700 to 2900 1/h is selected in the case of He/H₂=30:70, and a dew point of the gas flow before entry into the oven of −90° C. is selected. A dew point of less than 0° C. is thereby obtained at the gas outlet.

If a silicon dioxide granulate with a residual moisture of 0.05% is fed to the oven at 30 kg/h, a gas replacement rate of the gas flow in a range from 1900 to 2100 l/h is selected in the case of He/H₂=50:50 and in a range from 1800 to 2000 l/h is selected in the case of He/H₂=30:70, and a dew point of the gas flow before entry into the oven of −90° C. is selected. A dew point of less than 0° C. is thereby obtained at the gas outlet.

If a silicon dioxide granulate with a residual moisture of 0.03% is fed to the oven at 30 kg/h, a gas replacement rate of the gas flow in a range from 1400 to 1600 l/h is selected in the case of He/H₂=50:50 and in a range from 1200 to 1400 l/h is selected in the case of He/H₂=30:70, and a dew point of the gas flow before entry into the oven of −90° C. is selected. A dew point of less than 0° C. is thereby obtained at the gas outlet.

The oven temperature for melting the silicon dioxide granulate is preferably in the range from 1700 to 2500° C., for example in the range from 1900 to 2400° C., particularly preferably in the range from 2100 to 2300° C.

Preferably, the holding time in the oven is in a range from 1 hour to 50 hours, for example 1 to 30 hours, particularly preferably 5 to 20 hours. In the context of the present invention, the holding time means the time which is required, when carrying out the process according to the invention, to remove the fill quantity of the melting oven from the melting oven in which the glass melt is formed, in a manner according to the invention. The fill quantity is the entire mass of silicon dioxide in the melting oven. In this connection, the silicon dioxide can be present as a solid and as a glass melt.

Preferably, the oven temperature increases over the length in the direction of the material transport. Preferably, the oven temperature increases over the length in the direction of the material transport by at least 100° C., for example by at least 300° C. or by at least 500° C. or by at least 700° C., particularly preferably by at least 1000° C. Preferably, the maximum temperature in the oven is 1700 to 2500° C., for example 1900 to 2400° C., particularly preferably 2100 to 2300° C. The increase of the oven temperature can proceed uniformly or according to a temperature profile.

Preferably, the oven temperature decreases before the glass melt is removed from the oven. Preferably, the oven temperature decreases before the glass melt is removed from the oven by 50 to 500° C., for example by 100° C. or by 400° C., particularly preferably by 150 to 300° C. Preferably, the temperature of the glass melt on removal is 1750 to 2100° C., for example 1850 to 2050° C., particularly preferably 1900 to 2000° C.

Preferably, the oven temperature increases over the length in the direction of the material transport and decreases before the glass melt is removed from the oven. In this connection, the oven temperature preferably increases over the length in the direction of the material transport by at least 100° C., for example by at least 300° C. or by at least 500° C. or by at least 700° C., particularly preferably by at least 1000° C. Preferably, the maximum temperature in the oven is 1700 to 2500° C., for example 1900 to 2400° C., particularly preferably 2100 to 2300° C. Preferably, the oven temperature decreases before the glass melt is removed from the oven by 50 to 500° C., for example by 100° C. or by 400° C., particularly preferably by 150 to 300° C.

Pre-Heating Section

Preferably, the oven has at least a first and a further chamber joined to each other by a passage, the first and the second chamber having a different temperature, the temperature of the first chamber being lower than the temperature of the further chamber. In the further chamber, a glass melt is formed from the silicon dioxide granulate. This chamber is referred to as melting chamber in the following. A chamber which is joined to the melting chamber via a duct but which is upstream of it is also referred to as pre-heating section. An example is those in which at least one outlet is directly connected with the inlet of the melting chamber. The arrangement above may also be made in independent ovens, in which case the melting chamber is a melting oven. In the further description, however, the term ‘melting oven’ may be taken as being identical to the term ‘melting chamber’: so what is said concerning the melting oven may also be taken as applying to the melting chamber and vice versa. The term ‘preheating section’ means the same in both cases.

Preferably, the silicon dioxide granulate has a temperature in a range from 20 to 1300° C. on entry into the oven.

According to a first embodiment, the silicon dioxide granulate is not tempered prior to entry into the melting chamber. The silicon dioxide granulate has for example a temperature in a range from 20 to 40° C. on entry into the oven, particularly preferably from 20 to 30° C. If silicon dioxide granulate II is provided according to step i.), it preferably has a temperature on entry into the oven in a range from 20 to 40° C., particularly preferably from 20 to 30° C.

According to another embodiment, the silicon dioxide granulate is tempered up to a temperature in a range from 40 to 1300° C. prior to entry into the oven. Tempering means setting the temperature to a selected value. The tempering can in principle be carried out in any way known to the skilled person and known for the tempering of silicon dioxide granulate. For example, the tempering can be carried out in an oven arranged separate from the melting chamber or in an oven connected to the melting chamber.

Preferably, the tempering is carried out in a chamber connected to the melting chamber. Preferably, the oven therefore comprises a pre-heating section in which the silicon dioxide can be tempered. Preferably, the pre-heating section is itself a feed oven, particularly preferably a rotary kiln. A feed oven means a heated chamber which, in operation, effects a movement of the silicon dioxide from an inlet of the feed oven to an outlet of the feed oven. Preferably, the outlet is directly connected to an inlet of the melting oven. In this way, the silicon dioxide granulate can arrive from the pre-heating section into the melting oven without further intermediate steps or means.

It is further preferred that the pre-heating section comprises at least one gas inlet and at least one gas outlet. Through the gas inlet, the gas can arrive in the interior, the gas chamber of the pre-heating section, and through the gas outlet it can be removed. It is also possible to introduce gas into the pre-heating section via the inlet of the pre-heating section for the silicon dioxide granulate. Also, gas can be removed via the outlet of the pre-heating section and subsequently separated from the silicon dioxide granulate. Furthermore, preferably, the gas can be introduced via the inlet for the silicon dioxide granulate and a gas inlet of the pre-heating section, and removed via the outlet of the pre-heating section and a gas outlet of the pre-heating section.

Preferably, a gas flow is produced in the pre-heating section by use of the gas inlet and of the gas outlet. Suitable gases are for example hydrogen, inert gases as well as two or more thereof. Preferred inert gases are nitrogen, helium, neon, argon, krypton and xenon, particularly preferably nitrogen and helium. Preferably, a reducing atmosphere is present in the pre-heating section. This can be provided in the form of hydrogen or a combination of hydrogen and an inert gas, for example a combination of hydrogen and helium or of hydrogen and nitrogen, particularly preferably a combination of hydrogen and helium. Furthermore, preferably, an oxidising atmosphere is present in the pre-heating section. This can preferably be provided in the form of oxygen or a combination of oxygen and one or more further gases, air being particularly preferable. Further preferably, it is possible of the silicon dioxide to be tempered at reduced pressure in the pre-heating section.

For example, the silicon dioxide granulate can have a temperature on entry into the oven in a range from 100 to 1100° C. or from 300 to 1000 or from 600 to 900° C. If silicon dioxide granulate II is provided according to step i.), it preferably has a temperature on entry into the oven in a range from 100 to 1100° C. or from 300 to 1000 or from 600 to 900° C.

According to a preferred embodiment of the first aspect of the invention, the oven comprises at least two chambers. Preferably, the oven comprises a first and at least one further chamber. The first and the further chamber are connected to each other by a passage.

The at least two chambers can in principle be arranged in the oven in any manner, preferably vertical or horizontal, particularly preferably vertical. Preferably, the chambers are arranged in the oven in such a way that on carrying out the process according to the first aspect of the invention, silicon dioxide granulate passes through the first chamber and is subsequently heated in the further chamber to obtain a glass melt. The further chamber preferably has the above described features of the melting oven and of the crucible arranged therein.

Preferably, each of the chambers comprises an inlet and an outlet. Preferably, the inlet of the oven is connected to the inlet of the first chamber via a passage. Preferably, the outlet of the oven is connected to the outlet of the further chamber via a passage. Preferably, the outlet of the first chamber is connected to the inlet of the further chamber via a passage.

Preferably, the chambers are arranged in such a manner that the silicon dioxide granulate can arrive in the first chamber through the inlet of the oven. Preferably, the chambers are arranged in such a manner that a silicon dioxide glass melt can be removed from the further chamber through the outlet of the oven. Particularly preferably, the silicon dioxide granulate can arrive in the first chamber through the inlet of the oven and a silicon dioxide glass melt can be removed from a further chamber through the outlet of the oven.

The silicon dioxide, in the form of granulate or powder, can go from a first into a further chamber through the passage in the direction of material transport as defined by the process. Reference to chambers connected by a passage includes arrangements in which further intermediate elements arrange in the direction of the material transport between a first and a further chamber. In principle, gases, liquids and solids can pass through the passage. Preferably, silicon dioxide powder, slurries of silicon dioxide powder and silicon dioxide granulate can pass through the passage between a first and a further chamber. Whilst the process according to the invention is carried out, all of the materials introduced into the first chamber can arrive in the further chamber via the passage between the first and the further chamber. Preferably, only silicon dioxide in the form of granulate or powder arrive in the further chamber via the passage between the first and the further chamber. Preferably, the passage between the first and the further chamber is closed up by the silicon dioxide, such that the gas chamber of the first and the further chamber are separated from each other, preferably such that in different gases or gas mixtures, different pressures or both can be present in the gas chambers. According to another preferred embodiment, the passage is formed of a gate, preferably a rotary gate valve.

Preferably, the first chamber of the oven has at least one gas inlet and at least one gas outlet. The gas inlet can in principle have any form which is known to the skilled person and which is suitable for introduction of a gas, for example a nozzle, a vent or a tube. The gas outlet can in principle have any form known to the skilled person and which is suitable for removal of a gas, for example a nozzle, a vent or a tube.

Preferably, silicon dioxide granulate is introduced into the first chamber through the inlet of the oven and warmed. The warming can be carried out in the presence of a gas or of a combination of two or more gases. To this end, the gas or the combination of two or more gases is present in the gas chamber of the first chamber. The gas chamber of the first chamber means the region of the first chamber which is not occupied by a solid or liquid phase. Suitable gases are for example hydrogen, oxygen, inert gases as well as two or more thereof. Preferred inert gases are nitrogen, helium, neon, argon, krypton and xenon, particularly preferred are nitrogen, helium and a combination thereof. Preferably, the warming is carried out in a reducing atmosphere. This can preferably be provided in the form of hydrogen or a combination of hydrogen and helium. Preferably, the silicon dioxide granulate is warmed in the first chamber in a flow of the gas or of the combination of two or more gases.

It if further preferred that the silicon dioxide granulate is warmed in the first chamber at reduced pressure, for example at a pressure of less than 500 mbar or less than 300 mbar, for example 200 mbar or less.

Preferably, the first chamber has at least one device with which the silicon dioxide granulate is moved. In principle, all devices can be selected which are known to the skilled person for this purpose and which appear suitable. Preferably, stirring, shaking and slewing devices.

According to a preferred embodiment of the first aspect of the invention, the temperatures in the first and in the further chamber are different. Preferably, the temperature in the first chamber is lower than the temperature in the further chamber. Preferably, the temperature difference between the first and the further chamber is in a range from 600 to 2400° C., for example in a range from 1000 to 2000° C. or from 1200 to 1800° C., particularly preferably in a range from 1500 to 1700° C. Furthermore, preferably, the temperature the first chamber is 600 to 2400° C., for example 1000 to 2000° C. or 1200 to 1800° C., particularly preferably 1500 to 1700° C. lower than the temperature in the further chamber.

According to a preferred embodiment, the first chamber of the oven is a pre-heating section, particularly preferably a pre-heating section as described above, which has the features as described above. Preferably, the pre-heating section is connected to the further chamber via a passage. Preferably, silicon dioxide goes from the pre-heating section via a passage into the further chamber. The passage between the pre-heating section and the further chamber can be closed off, so that no gases introduced into the pre-heating section go through the passage into the further chamber. Preferably, the passage between the pre-heating section and the further chamber is closed off, so that the silicon dioxide does not come into contact with water. The passage between the pre-heating section and the further chamber can be closed off, so that the gas chamber of the pre-heating section and the first chamber are separated from each other in such a way that different gases or gas mixtures, different pressures or both can be present in the gas chambers. A suitable passage is preferably as per the above described embodiments.

According to a further preferred embodiment, the first chamber of the oven is not a pre-heating section. For example, the first chamber can be a levelling chamber. A levelling chamber is a chamber of the oven in which variations in throughput in a pre-heating section upstream thereof, or throughput differences between a pre-heating section and the further chamber are levelled. For example, as described above a rotary kiln can be arranged upstream of the first chamber. This commonly has a throughput which can vary by an amount up to 6% of the average throughput. Preferably, silicon dioxide is held in a levelling chamber at the temperature at which it arrives in the levelling chamber.

It is also possible for the oven to have a first chamber and more than one further chambers, for example two further chambers or three further chambers or four further chambers or five further chambers or more than five further chambers, particularly preferably two further chambers. If the oven has two further chambers, the first chamber is preferably a pre-heating section, the first of the further chambers a levelling chamber and the second of the further chambers the melting chamber, based on the direction of the material transport.

According to a further preferred embodiment, an additive is present in the first chamber. The additive is preferably selected from the group consisting of halogens, inert gases, bases, oxygen or a combination of two or more thereof.

In principle, halogens in elemental form and halogen compounds are suitable additives. Preferred halogens are selected from the group consisting of chlorine, fluorine, chlorine containing compounds and fluorine containing compounds. Particularly preferable are elemental chlorine and hydrogen chloride.

In principle, all inert gases as well as mixtures of two or more thereof are suitable additives. Preferred inert gases are nitrogen, helium or a combination thereof.

In principle bases are also suitable additives. Preferred bases for use as additives are inorganic and organic bases.

Further, oxygen is a suitable additive. The oxygen is preferably present as an oxygen containing atmosphere, for example in combination with an inert gas or a mixture of two or more inert gases, particularly preferably in combination with nitrogen, helium or nitrogen and helium.

The first chamber can in principle comprise any material which is known to the skilled person and which is suitable for heating silicon dioxide. Preferably, the first chamber comprises at least one element selected from the group consisting of quartz glass, a refractory metal, aluminium and a combination of two or more thereof, particularly preferably, the first chamber comprises quartz glass or aluminium.

Preferably, the temperature in the first chamber does not exceed 600° C. if the first chamber comprises a polymer or aluminium. Preferably, the temperature in the first chamber is 100 to 1100° C., if the first chamber comprises quartz glass. Preferably, the first chamber comprises mainly quartz glass.

In the transportation of the silicon dioxides from the first chamber to the further chamber through the passage between the first and the further chamber, the silicon dioxide can in principle be present in any state. Preferably, the silicon dioxide is present as a solid, for example as particles, powder or granulate. According to a preferred embodiment of the first aspect of the invention, the transportation of the silicon dioxides from the first to the further chamber is carried out as granulate.

According to a further preferred embodiment, the further chamber is a crucible made of a metal sheet or of a sinter material, wherein the sinter material comprises a sinter metal, wherein the metal sheet or the sinter metal is selected from the group consisting of molybdenum, tungsten and a combination thereof.

The glass melt is removed from the oven through the outlet, preferably via a nozzle.

Step iii.)

A quartz glass body is made out of at least a part of the glass melt. For this, preferably at least a part of the glass melt made in step ii) is removed and the quartz glass body is made out of it.

The removal of the part of the glass melt made in step ii) can in principle be carried out continuously from the melting oven or the melting chamber or after the production of the glass melt has been finished. Preferably, a part of the glass melt is removed continuously. The glass melt is removed through the outlet of the oven or through the outlet of the melting chamber, in each case preferably via a nozzle.

The glass melt can be cooled before, during or after the removal, to a temperature which enables the forming of the glass melt. A rise in the viscosity of the glass melt is connected to the cooling of the glass melt. The glass melt is preferably cooled to such an extent that in the forming, the produced form holds and the forming is at the same time as easy and reliable as possible and can be carried out with little effort. The skilled person can easily establish the viscosity of the glass melt for forming by varying the temperature of the glass melt at the forming tool. Preferably, the glass melt has a temperature on removal in the range from 1750 to 2100° C., for example 1850 to 2050° C., particularly preferably 1900 to 2000° C. Preferably, the glass melt is cooled to a temperature of less than 500° C. after removal, for example of less than 200° C. or less than 100° C. or less than 50° C., particularly preferably to a temperature in the range from 20 to 30° C.

The quartz glass body which is formed can be a solid body or a hollow body. A solid body means a body which is mainly made out of a single material. Nevertheless, a solid body can have one or more inclusions, e.g. gas bubbles. Such inclusions in a solid body commonly have a size of 65 mm³ or less, for example of less than 40 mm³, or of less than 20 mm³, or of less than 5 mm³, or of less than 2 mm³, particularly preferably of less than 0.5 mm³. Preferably, a solid body comprises less than 0.02 vol.-% of its volume as inclusion, for example less than 0.01 vol.-% or less than 0.001 vol.-%, in each case based on the total volume of the solid body.

The quartz glass body has an exterior form. The exterior form means the form of the outer edge of the cross section of the quartz glass body. The exterior form of the quartz glass body in cross-section is preferably round, elliptical or polygonal with three or more corners, for example 4, 5, 6, 7 or 8 corners, particularly preferably the quartz glass body is round.

Preferably, the quartz glass body has a length in the range from 100 to 10000 mm, for example from 1000 to 4000 mm, particularly preferably from 1200 to 3000 mm.

Preferably, the quartz glass body has an exterior diameter in the range from 1 to 500 mm, for example in a range from 2 to 400 mm, particularly preferably in a range from 5 to 300 mm.

The forming of the quartz glass body is performed by means of a nozzle. The glass melt is sent through the nozzle. The exterior form of a quartz glass body formed through the nozzle is determined by the form of the nozzle opening. If the opening is round, a cylinder will be made in forming the quartz glass body. If the opening of the nozzle has a structure, this structure will be transferred to the exterior form of the quartz glass body. A quartz glass body which is made by means of a nozzle with structure at the opening, has an image of the structure in the length direction along the glass strand.

The nozzle is integrated in the melting oven. Preferably, it is integrated in the melting oven as part of the crucible, particularly preferably as part of the outlet of the crucible.

Preferably, at least a part of the glass melt is removed through the nozzle. The exterior form of the quartz glass body is formed by the removal of the at least part of the glass melt through the nozzle.

Preferably, the quartz glass body is cooled after the forming, so that it maintains its form. Preferably, the quartz glass body is cooled after the forming to a temperature which is at least 1000° C. below the temperature of the glass melt in the forming, for example at least 1500° C. or at least 1800° C., particularly preferably 1900 to 1950° C. Preferably, the quartz glass body is cooled to a temperature of less than 500° C., for example of less than 200° C. or less than 100° C. or less than 50° C., particularly preferably to a temperature in the range from 20 to 30° C.

According to a preferred embodiment of the first aspect of the invention, the obtained quartz glass body can be treated with at least one procedure selected from the group consisting of chemical, thermal or mechanical treatment.

Preferably, the quartz glass body is chemically post treated. Post treatment relates to treatment of a quartz glass body which has already been made. Chemical post treatment of the quartz glass body means in principle any procedure which is known to the skilled person and appears suitable for employing materials for changing the chemical structure or the composition of the surface of the quartz glass body, or both. Preferably, the chemical post treatment comprises at least one means selected from the group consisting of treatment with fluorine compounds and ultrasound cleaning.

Possible fluorine compounds are in particular hydrogen fluoride and fluorine containing acids, for example hydrofluoric acid. The liquid preferably has a content of fluorine compounds in a range from 35 to 55 wt.-%, preferably in a range from 35 to 45 wt.-%, the wt.-% in each case based on the total amount of liquid. The remainder up to 100 wt.-% is usually water. Preferably, the water is fully desalinated water or deionised water.

Ultrasound cleaning is preferably performed in a liquid bath, particularly preferably in the presence of detergents. In the case of ultrasound cleaning, commonly no fluorine compounds, for example neither hydrofluoric acid nor hydrogen fluoride.

The ultrasound cleaning of the quartz glass body is preferably carried out under at least one, for example at least two or at least three or at least four or at least five, particularly preferably all of the following conditions:

-   -   The ultrasound cleaning performed in a continuous process.     -   The equipment for the ultrasound cleaning has six chambers         connected to each other by tubes.     -   The holding time for the quartz glass body in each chamber can         be set. Preferably, the holding time of the quartz glass body in         each chamber is the same. Preferably, the holding time in each         chamber is in a range from 1 to 120 min, for example of less         than 5 min or from 1 to 5 min or from 2 to 4 min or of less than         60 min or from 10 to 60 min or from 20 to 50 min, particularly         preferably in a range from 5 to 60 min.     -   The first chamber comprises a basic medium, preferably         containing water and a base, and an ultrasound cleaner.     -   The third chamber comprises an acidic medium, preferably         containing water and an acid, and an ultrasound cleaner.     -   In the second chamber and in the fourth to sixth chamber, the         quartz glass body is cleaned with water, preferably with         desalinated water.     -   The fourth to sixth chambers are operated with cascades of         water. Preferably, the water is only introduced in the sixth         chamber and funs from the sixth chamber into the fifth and from         the fifth chamber into the fourth.

Preferably, the quartz glass body is thermally post treated. Thermal post treatment of the quartz glass body means in principle a procedure known to the skilled person and which appears suitable for changing the form or structure or both of the quartz glass body by means of temperature. Preferably, the thermal post treatment comprises at least a one means selected from the group consisting of tempering, compressing, inflating, drawing, welding, and a combination of two or more thereof. Preferably, the thermal post treatment is not performed for the purpose of removing material.

The tempering is preferably performed by heating the quartz glass body in an oven, preferably at a temperature in a range from 900 to 1300° C., for example in a range from 900 to 1250° C. or from 1040 to 1300° C., particularly preferably in a range from 1000 to 1050° C. or from 1200 to 1300° C. Preferably, in the thermal treatment, a temperature of 1300° C. is not exceeded for continuous period of more than 1 h, particularly preferably a temperature of 1300° C. is not exceeded during the entire duration of the thermal treatment. The tempering can in principle be performed at reduced pressure, at normal pressure or at increased pressure, preferably at reduced pressure, particularly preferably in a vacuum.

The compressing is preferably performed by heating the quartz glass body, preferably to a temperature of about 2100° C., and subsequent forming during a rotating turning motion, preferably with a rotation speed of about 60 rpm. For example, a quartz glass body in the form of a rod can be formed into a cylinder.

Preferably, a quartz glass body can be inflated by injecting a gas into the quartz glass body. For example, a quartz glass body can by formed into a large-diameter tube by inflating. For this, preferably the quartz glass body is heated to a temperature of about 2100° C., whilst a rotating turning motion is performed, preferably with a rotation speed of about 60 rpm, and the interior is flushed with a gas, preferably at a defined and controlled inner pressure of up to about 100 mbar. A large-diameter tube means a tube with an outer diameter of at least 500 mm.

A quartz glass body can preferably be drawn. The drawing is preferably performed by heating the quartz glass body, preferably to a temperature of about 2100° C., and subsequently pulling with a controlled pulling speed to the desired outer diameter of the quartz glass body. For example lamp tubes can be formed from quartz glass bodies by drawing.

Preferably, the quartz glass body is mechanically post treated. A mechanic post treatment of the quartz glass body means in principle any procedure known to the skilled person and which appears suitable for using an abrasive means to change the shape of the quartz glass body or to split the quartz glass body into multiple pieces. In particular, the mechanical post treatment comprises at least one means selected from the group consisting of grinding, drilling, honing, sawing, waterjet cutting, laser cutting, roughening by sandblasting, milling and a combination of two or more thereof.

Preferably, the quartz glass body is treated with a combination of these procedures, for example with a combination of a chemical and a thermal post treatment or a chemical and a mechanical post treatment or a thermal and a mechanical post treatment, particularly preferably with a combination of a chemical, a thermal and a mechanical post treatment. Furthermore, preferably, the quartz glass body can be subjected to several of the above mentioned procedures, each independently from the others.

Pre-Compacting

It is in principle possible to subject the silicon dioxide granulate provided in step i.) to one or multiple pretreatment steps, before it is warmed in step ii.) to obtain a glass melt. Possible pre-treatment steps are for example thermal or mechanical treatment steps. For example the silicon dioxide granulate can be compactified before the warming in step ii.). “Compacting” means a reduction in the BET surface area and a reduction of the pore volume.

The silicon dioxide granulate is preferably compactified by heating the silicon dioxide granulate or mechanically by exerting a pressure to the silicon dioxide granulate, for example rolling or pressing of the silicon dioxide granulate. Preferably, the silicon dioxide granulate is compactified by heating. Particularly preferably, the compacting of the silicon dioxide granulate is performed by heating by means of a pre-heating section which is connected to the melting oven.

Preferably, the silicon dioxide is compactified by heating at a temperature in a range from 800 to 1400° C., for example at a temperature in a range from 850 to 1300° C., particularly preferably at a temperature in a range from 900 to 1200° C.

In a preferred embodiment of the first aspect of the invention, the BET surface area of the silicon dioxide granulate is not reduced to less than 5 m²/g prior to the warming in step ii.), preferably not to less than 7 m²/g or not to less than 10 m²/g, particularly preferably not to less than 15 m²/g. Furthermore, it is preferred, that the BET surface area of the silicon dioxide granulate is not reduced prior to the warming in step ii.) compared with the silicon dioxide granulate provided in step i.).

In a preferred embodiment of the first aspect of the invention, the BET surface area of the silicon dioxide granulate is reduced to less than 20 m²/g, for example to less than 15m²/g, or to less than 10 m²/g, or to a range of more than 5 to less than 20 m²/g or from 7 to 15 m²/g, particularly preferably to a range of 9 to 12 m²/g. Preferably, the BET surface area of the silicon dioxide granulate is reduced prior to the heating in step ii.) in comparison to the silicon dioxide granulate provided in step i.) by less than 40 m²/g, for example by 1 to 20 m²/g or by 2 to 10 m²/g, particularly preferably by 3 to 8 m²/g, the BET surface area after the compactification being more than 5 m²/g.

The compactified silicon dioxide granulate is referred to in the following as silicon dioxide granulate III. Preferably, the silicon dioxide granulate III has at least one, for example at least two or at least three or at least four, particularly preferably at least five of the following features:

-   -   A. a BET surface area in a range of more than 5 to less than 40         m²/g, for example from 10 to 30 m²/g, particularly preferably in         a range of 15 to 25 m²/g;     -   B. a particle size D₁₀ in a range from 100 to 300 μm,         particularly preferably in a range from 120 to 200 μm;     -   C. a particle size D₅₀ in a range from 150 to 550 μm,         particularly preferably in a range from 200 to 350 μm;     -   D. a particle size D₉₀ in a range from 300 to 650 μm,         particularly preferably in a range from 400 to 500 μm;     -   E. a bulk density in a range from 0.8 to 1.6 g/cm³, particularly         preferably from 1.0 to 1.4 g/cm³;     -   F. a tamped density in a range from 1.0 to 1.4 g/cm³,         particularly preferably from 1.15 to 1.35 g/cm³;     -   G. a carbon content of less than 5 ppm, for example of less than         4.5 ppm, particularly preferably of less than 4 ppm;     -   H. a Cl content of less than 500 ppm, particularly preferably         from 1 ppb to 200 ppm, wherein the ppm and ppb are each based on         the total weight of the silicon dioxide granulate III.

The silicon dioxide granulate III preferably has the feature combination A./F./G. or A./F./H. or A./G./H., further preferably the feature combination A./F./G./H.

The silicon dioxide granulate III preferably has the feature combination A./F./G., wherein the BET surface area is in a range from 10 to 30 m²/g, the tamped density is in a range from 1.15 to 1.35 g/mL and the carbon content is less than 4 ppm.

The silicon dioxide granulate III preferably has the feature combination A./F./H., wherein the BET surface area is in a range from 10 to 30 m²/g, the tamped density is in a range from 1.15 to 1.35 g/mL and the chlorine content is in a range from 1 ppb to 200 ppm.

The silicon dioxide granulate III preferably has the feature combination A./G./H., wherein the BET surface area is in a range from 10 to 30 m²/g, the carbon content is less than 4 ppm and the chlorine content is in a range from 1 ppb to 200 ppm.

The silicon dioxide granulate III preferably has the feature combination A./F./G./H., wherein the BET surface area is in a range from 10 to 30 m²/g, the tamped density is in a range from 1.15 to 1.35 g/mL, the carbon content is less than 4 ppm and the chlorine content is in a range from 1 ppb to 200 ppm.

Preferably, in at least one process step, a silicon component different to silicon dioxide is introduced. The introduction of silicon components different to silicon dioxide is also referred to in the following as Si-doping. In principle, the Si-doping can be performed in any process step. Preferably, the Si-doping is preferred in step i.) or in step ii.).

The silicon component which is different to silicon dioxide can in principle be introduced in any form, for example as a solid, as a liquid, as a gas, in solution or as a dispersion. Preferably, the silicon component different to silicon dioxide is introduced as a powder. Also, preferably, the silicon component different to silicon dioxide can be introduced as a liquid or as a gas.

The silicon component which is different to silicon dioxide is preferably introduced in an amount in a range from 1 to 100,000 ppm, for example in a range from 10 to 10,000 ppm or from 30 to 1000 ppm or in a range from 50 to 500 ppm, particularly preferably in a range from 80 to 200 ppm, further particularly preferably in a range from 200 to 300 ppm, in each case based on the total weight of silicon dioxide.

The silicon component which is different to silicon dioxide can be solid, liquid or gaseous. If it is solid, it preferably has a mean particle size of up to 10 mm, for example of up to 1000 μm of up to 400 μm or in a range from 1 to 400 μm, for example 2 to 200 μm or 3 to 100 μm, particularly preferably in a range from 5 to 50 μm. The particle size values are based on the state of the silicon component which is different to silicon dioxide at room temperature.

The silicon component preferably has a purity of at least 99.5 wt.-%, for example at least 99.8 wt.-% or at least 99.9 wt.-%, particularly preferably at least 99.94 wt.-%, in each case based on the total weight of the silicon component. Preferably, the silicon component has a carbon content of not more than 1000 ppm, for example not more than 700 ppm, particularly preferably not more than 500 ppm, in each case based on the total weight the silicon component. Particularly preferably, this applies to silicon employed as the silicon component. Preferably, the silicon component has an amount of impurities selected from the group consisting of Al, Ca, Co, Cr, Cu, Fe, Ge, Hf, K, Li, Mg, Mn, Mo, Na, Nb, Ni, Ti, V, W, Zn, Zr of not more than 250 ppm, for example not more than 150 ppm, particularly preferably not more than 100 ppm, in each case based on the total weight of the silicon component. Particularly preferably, this applies where silicon is employed as the silicon component.

Preferably, a silicon component which is different to silicon dioxide is introduced in process step i.). Preferably, the silicon component which is different to silicon dioxide is introduced during the processing of the silicon dioxide powder to obtain a silicon dioxide granulate (step II.). For example the silicon component which is different to silicon dioxide can be introduced before, during or after the granulation.

Preferably, the silicon dioxide can be Si-doped by introducing the silicon component which is different to silicon dioxide to the slurry comprising silicon dioxide powder. For example, the silicon component which is different to silicon dioxide can be mixed with silicon dioxide powder and subsequently slurried, or the silicon component which is different to silicon dioxide can be introduced into a slurry of silicon dioxide powder a liquid and then slurried, or the silicon dioxide powder can be introduced into a slurry or solution of the silicon component which is different to silicon dioxide in a liquid and then slurried.

Preferably, the silicon dioxide can be Si-doped by introduction of the silicon component which is different to silicon dioxide during granulation It is in principle possible to introduce the silicon component which is different to silicon dioxide at any point in time during the granulation. In the case of spray granulation, the silicon component which is different to silicon dioxide can for example be sprayed through the nozzle into the spray tower together with the slurry comprising silicon dioxide powder. In the case of roll granulation, the silicon component which is different to silicon dioxide can preferably be introduced in solid form or as a slurry comprising silicon dioxide powder, for example after introducing the slurry comprising silicon dioxide powder into the stirring vessel.

Furthermore, preferably, the silicon dioxide can be Si-doped by introduction of the silicon component which is different to silicon dioxide after the granulation. For example, the silicon dioxide can be doped during the treatment of the silicon dioxide granulate Ito obtain silicon dioxide granulate II, preferably by introducing the silicon component which is different to silicon dioxide during the thermal or mechanical treatment of the silicon dioxide granulate I.

Preferably, the silicon dioxide granulate II is doped with the silicon component which is different to silicon dioxide.

Furthermore, preferably, the silicon component which is different to silicon dioxide can also be introduced during several of the above mentioned sections, in particular during and after the thermal or mechanical treatment of the silicon dioxide granulate Ito obtain the silicon dioxide granulate II.

The silicon component which is different to silicon dioxide can in principle be silicon or any silicon containing compound known to the skilled person and which has a reducing effect. Preferably, the silicon component which is different to silicon dioxide is silicon, a silicon-hydrogen compound, for example a silane, a silicon-oxygen compound, for example silicon monoxide, or a silicon-hydrogen-oxygen compound, for example disiloxane. Examples of preferred silanes are monosilane, disilane, trisilane, tetrasilane, pentasilane, hexasilane, heptasilane, higher homologous compounds as well as isomers of the aforementioned, and cyclic silanes like cyclopentasilane

Preferably, a silicon component which is different to silicon dioxide is introduced in process step ii.).

Preferably, the silicon component which is different to silicon dioxide can be introduced directly into the melting crucible with the silicon dioxide granulate. Preferably, silicon as the silicon component which is different from silicon dioxide can be introduced into the melting crucible with the silicon dioxide granulate. The silicon is preferably introduced as powder, in particular with the particle size previously given for the silicon component which is different to silicon dioxide.

Preferably, the silicon component which is different to silicon dioxide is added to the silicon dioxide granulate before introduction into the melting crucible. The addition can in principle be performed at any time after the formation of the granulate, for example in the pre-heating section, before or during the pre-compacting of the silicon dioxide granulate II, or to the silicon dioxide granulate III.

A silicon dioxide granulate obtained by addition of a silicon component which is different to silicon dioxide is referred to in the following as “Si-doped granulate”. Preferably, the Si-doped granulate has at least one, for example at least two or at least three or at least four, particularly preferably at least five of the following features:

-   -   [1] a BET surface area in a range of more than 5 to less than 40         m²/g, for example from 10 to 30 m²/g, particularly preferably in         a range from 15 to 25 m²/g;     -   [2] a particle size D₁₀ in a range from 100 to 300 μm,         particularly preferably in a range from 120 to 200 μm;     -   [3] a particle size D₅₀ in a range from 150 to 550 μm,         particularly preferably in a range from 200 to 350 μm;     -   [4] a particle size D₉₀ in a range from 300 to 650 μm,         particularly preferably in a range from 400 to 500 μm;     -   [5] a bulk density in a range from 0.8 to 1.6 g/cm³,         particularly preferably from 1.0 to 1.4 g/cm³;     -   [6] a tamped density in a range from 1.0 to 1.4 g/cm³,         particularly preferably from 1.15 to 1.35 g/cm³;     -   [7] a carbon content of less than 5 ppm, for example of less         than 4.5 ppm, particularly preferably of less than 4 ppm;     -   [8] a Cl content of less than 500 ppm, particularly preferably         from 1 ppb to 200 ppm;     -   [9] an Al content of less than 200 ppb, particularly preferably         from 1 ppb to 100 ppb;     -   [10] a metal content of metals which are different to aluminium,         of less than 1000 ppb, for example in a range from 1 to 400 ppb,         particularly preferably in a range from 1 to 200 ppb;     -   [11] a residual moisture content of less than 3 wt.-%, for         example in a range from 0.001 wt.-% to 2 wt.-%, particularly         preferably from 0.01 to 1 wt.-%;     -   wherein the wt.-%, ppm and ppb are each based on the total         weight of the Si-doped granulate.

In a preferred embodiment of the first aspect of the invention, the melting energy is transferred to the silicon dioxide granulate via a solid surface.

A solid surface means a surface which is different to the surface of the silicon dioxide granulate and which does not melt or collapse at the temperatures to which the silicon dioxide granulate is heated for melting. Suitable materials for the solid surface are for example the materials which are suitable as crucible material.

The solid surface can in principle be any surface which is known to the skilled person and which is suitable for this purpose. For example the crucible or a separate component which is not the crucible can be used as the solid surface.

The solid surface can in principle be heated in any manner known to the skilled person and which is suitable for this purpose, in order to transfer the melting energy to the silicon dioxide granulate. Preferably, the solid surface is heated by resistive heating or inductive heating. In the case of inductive heating, the energy is directly coupled into the solid surface by means of coils and delivered from there to its inner side. In the case of resistive heating, the solid surface is warmed from the outer side and passes the energy from there to its inner side. In this connection, a heating chamber gas with low heat capacity is advantageous, for example an argon atmosphere or an argon containing atmosphere. For example, the solid surface can be heated electrically or also by firing the solid surface with a flame from the outside. Preferably, the solid surface is heated to a temperature which can transfer an amount of energy to the silicon dioxide granulate and/or part melted silicon dioxide granulate which is sufficient for melting the silicon dioxide granulate.

If a separate component is used as the solid surface, this can be brought into contact with the silicon dioxide granulate in any manner, for example by laying the component on the silicon dioxide granulate or by introducing the component between the granules of the silicon dioxide granulate or by inserting the component between the crucible and the silicon dioxide granulate or by a combination of two or more thereof. The component can be heated before, or during or before and during the transfer of the melting energy.

Preferably, the melting energy is transferred to the silicon dioxide granulate via the inner side of the crucible. In this case, the crucible is heated enough so that the silicon dioxide granulate melts. The crucible is preferably heated resistively or inductively. The warmth is transferred from the outer side to the inner side of the crucible. The solid surface of the inner side of the crucible transfers the melting energy to the silicon dioxide granulate.

According to a further preferred embodiment of the present invention, the melting energy is not transferred to the silicon dioxide granulate via a gas compartment. Furthermore, preferably, the melting energy is not transferred to the silicon dioxide granulate by firing of the silicon dioxide granulate with a flame. Examples of these excluded means of transferring energy are directing one or multiple burner flames from above in the melting crucible, or onto the silicon dioxide, or both.

The above described process according to the first aspect of the invention relates to the preparation of a quartz glass body.

Preferably, the quartz glass body has at least one of the following features, for example at least two or at least three or at least four, particularly preferably at least five of the following features:

-   -   A] an OH content of less than 500 ppm, for example of less than         400 ppm, particularly preferably of less than 300 ppm;     -   B] a chlorine content of less than 60 ppm, preferably of less         than 40 ppm, for example of less than 40 ppm or less than 2 ppm         or less than 0.5 ppm, particularly preferably of less than 0.1         ppm;     -   C] an aluminium content of less than 200 ppb, for example of         less than 100 ppb, particularly preferably of less than 80 ppb;     -   D] an ODC content of less than 5·10¹⁵/cm³, for example in a         range from 0.1·10¹⁵ to 3·10¹⁵/cm³, particularly preferably in a         range from 0.5·10¹⁵ to 2.0·10¹⁵/cm³;     -   E] a metal content of metals which are different to aluminium,         of less than 1 ppm, for example of less than 0.5 ppm,         particularly preferably of less than 0.1 ppm;     -   F] a viscosity (p=1013 hPa) in a range from log₁₀ (η(1250°         C.)/dPas)=11.4 to log₁₀ (η(1250° C.)/dPas)=12.9 and/or log₁₀         (η(1300° C.)/dPas)=11.1 to log₁₀ (η(1300° C.) / dPas)=12.2         and/or log₁₀ (η(1350° C.)/dPas)=10.5 to log₁₀ (η(1350°         C.)/dPas)=11.5;     -   G] a standard deviation of the OH content of not more than 10%,         preferably not more than 5%, based on the OH content A] of the         quartz glass body;     -   H] a standard deviation of the chlorine content of not more than         10%, preferably not more than 5%, based on the chlorine content         B] of the quartz glass body;     -   I] a standard deviation of the aluminium content of not more         than 10%, preferably not more than 5%, based on the aluminium         content C] of the quartz glass body;     -   J] a refractive index homogeneity of less than 10⁻⁴;     -   K] a cylindrical form,     -   L] a tungsten content of less than 1000 ppb, for example of less         than 500 ppb or of less than 300 ppb or of less than 100 ppb or         in a range from 1 to 500 ppb or from 1 to 300 ppb, particularly         preferably in a range from 1 to 100 ppb;     -   M] a molybdenum content of less than 1000 ppb, for example of         less than 500 ppb or of less than 300 ppb or of less than 100         ppb or in a range from 1 to 500 ppb or from 1 to 300 ppb,         particularly preferably in a range from 1 to 100 ppb,     -   wherein the ppb and ppm are each based on the total weight of         the quartz glass body.

Preferably, the quartz glass body has a metal content of metals different to aluminium of less than 1000 ppb, for example of less than 500 ppb, particularly preferably of less than 100 ppb, in each case based on the total weight of the quartz glass body. Often however, the quartz glass body has a content of metals different to aluminium of at least 1 ppb. Such metals are for example sodium, lithium, potassium, magnesium, calcium, strontium, germanium, copper, molybdenum, titanium, iron and chromium. This can for example be present as an element, as an ion, or as part of a molecule or of an ion or of a complex.

The quartz glass body can comprise further constituents. Preferably, the quartz glass body comprises less than 500 ppm, for example less than 450 ppm, particularly preferably less than 400 ppm of further constituents, the ppm in each case being base on the total weight of the quartz glass body. Possible other constituents are for example carbon, fluorine, iodine, bromium and phosphorus. These can for example be present as an element, as an ion or as part of a molecule, an ion or a complex. Often however, the quartz glass body has a content of further constituents of at least 1 ppb.

Preferably, the quartz glass body comprises less than 5 ppm carbon, for example less than 4.5 ppm, particularly preferably less than 4 ppm, in each case based on the total weight of the quartz glass body. Often however, the quartz glass body has a carbon content of at least 1 ppb.

Preferably, the quartz glass body has a homogeneously distributed OH content, Cl content or Al content. An indicator of the homogeneity of the quartz glass body can be expressed as the standard deviation of OH content, Cl content or Al content. The standard deviation is the measure of the spread of the values of a variable from their arithmetic mean, here the OH content, chlorine content or aluminium content. For measuring the standard deviation, the content in the sample of the component in question e.g. OH, chlorine or aluminium, is measured at least seven measuring locations.

The quartz glass body preferably has the feature combination A]/B]/C] or A]/B]/D] or A]/B]/F], further preferred the feature combination A]/B]/C]/D] or A]/B]/]C]/F] or A]/B]/D]/F], further preferably the feature combination A]/B]/C]/D]/F].

The quartz glass body preferably has the feature combination A]/B]/C], wherein the OH content is less than 400 ppm, the chlorine content is less than 100 ppm and the aluminium content is less than 80 ppb.

The quartz glass body preferably has the feature combination A]/B]/D], the OH content is less than 400 ppm, the chlorine content is less than 100 ppm and the ODC content is in a range from 0.1·10¹⁵ to 3·10¹⁵/cm³.

The quartz glass body preferably has the feature combination A]/B]/F], wherein the OH content is less than 400 ppm, the chlorine content is less than 100 ppm and the viscosity (p=1013 hPa) is in a range from log₁₀ (η(1250° C.)/dPas)=11.4 to log₁₀ (η(1250° C.)/dPas)=12.9.

The quartz glass body preferably has the feature combination A]/B]/C]/D], wherein the OH content is less than 400 ppm, the chlorine content is less than 100 ppm, the aluminium content is less than 80 ppb and the ODC content is in a range from 0.1·10¹⁵ to 3·10¹⁵/cm³.

The quartz glass body preferably has the feature combination A]/B]/C]/F], wherein the OH content is less than 400 ppm, the chlorine content is less than 100 ppm, the aluminium content is less than 80 ppb and the viscosity (p=1013 hPa) is in a range from log₁₀ (η(1250° C.)/dPas)=11.4 to log₁₀ (η(1250° C.)/dPas)=12.9.

The quartz glass body preferably has the feature combination A]/B]/D]/F], wherein the OH content is less than 400 ppm, the chlorine content is less than 100 ppm, the ODC content is in a range from 0.1·10¹⁵ to 3·10¹⁵/cm³ and the viscosity (p=1013 hPa) is in a range from log₁₀ (η(1250° C.)/dPas)=11.4 to log₁₀ (η(1250° C.)/dPas)=12.9.

The quartz glass body preferably has the feature combination A]/B/C]/D]/F], wherein the OH content is less than 400 ppm, the chlorine content is less than 100 ppm, the aluminium content is less than 80 ppb and the ODC content is in a range from 0.1·10¹⁵ to 3·10¹⁵/cm³ and the viscosity (p=1013 hPa) is in a range from log₁₀ (η(1250° C.)/dPas)=11.4 to log₁₀ (η(1250° C.) / dPas)=12.9.

Step iv.)

According to the invention, as step iv.), the process comprises the reduction in size of the quartz glass body from step iii.). A quartz glass grain is obtained. Reducing the size of the quartz glass body can in principle take place in all ways which are known to the skilled man and suitable for this purpose. Preferably, the quartz glass body is mechanically or electromechanically reduced in size.

Particularly preferably, the quartz glass body is electromechanically reduced in size. Preferably, the electromechanical reduction in size takes place using high voltage discharge pulses, for example using an electric arc.

Preferably, the quartz glass body is reduced to particles with a particle size D₅₀ in a range from 50 μm to 5 mm. It is preferred to reduce the quartz glass body to a quartz glass grain with a particle size D₅₀ in a range from 50 μm to 500 μm, for example in a range from 75 to 350 μm, particularly preferably in a range from 100 to 250 μm. It is additionally preferred to reduce the quartz glass body to a quartz glass grain with a particle size D₅₀ in a range from 500 μm to 5 mm, for example in a range from 750 μm to 3.5 mm, particularly preferably in a range from 1 to 2.5 mm.

In the context of a further embodiment it is possible that the quartz glass body is firstly electromechanically, and then mechanically, further reduced in size. Preferably, the electromechanical reduction in size takes place as described previously. Preferably, the quartz glass body is reduced in size electromechanically to particles with a particle size D₅₀ of at least 2 mm, for example in a range from 2 to 50 mm or from 2.5 to 20 mm, particularly preferably in a range from 3 to 5 mm.

Preferably, the mechanical reduction in size takes place by pressure crushing, impact crushing, shearing or friction, preferably using crushers or grinders, for example using jaw crushers, rolling crushers, conical crushers, impact crushers, hammer crushers, shredders or ball mills, particularly preferably using jaw crushers or rolling crushers.

Preferably the quartz glass grain obtained after mechanical reduction in size has a particle size D₅₀ in a range from 50 μm to 5 mm.

Preferably, a quartz glass grain, the preparation of which comprises a mechanical reduction in size, is purified in a further processing step. Preferred purification measures are flotation steps, magnetic separation, acidification, preferably with HF, hot chlorination with HCl, Cl₂ or a combination thereof.

Preferably, the fines of the quartz glass grain are removed. The fines are removed for example by screening or sieving or a combination thereof. Quartz glass grain with a core size of less than 10 μm, for example less than 20 gm or of less than 30 μm, particularly preferably of less than 50 μm, are removed as fines.

Preferably, the quartz glass grain has at least one of the following features, for example at least two or at least three or at least four, particularly preferably at least five of the following features:

-   -   I/an OH content of less than 500 ppm, for example of less than         400 ppm, particularly preferably of less than 300 ppm;     -   II/a chlorine content of less than 60 ppm, preferably of less         than 40 ppm, for example of less than 40 ppm or less than 2 ppm         or less than 0.5 ppm, particularly preferably of less than 0.1         ppm;     -   III/an aluminium content of less than 1000 ppb, for example of         less than 500 ppb, particularly preferably of less than 200 ppb;     -   IV/a BET surface area of less than 1 m²/g, for example of less         than 0.5 m²/g, particularly preferably of less than 0.2 m²/g;     -   V/a bulk density in a range from 1.1 to 1.4 g/cm³, for example         in a range from 1.15 to 1.35 g/cm³, particularly preferably in a         range from 1.2 to 1.3 g/cm³;     -   VI/a particle size D₅₀ for melting in a range from 50 to 500 μm,         for example in a range from 75 to 350 μm, particularly         preferably in a range from 100 to 250 μm;     -   VII/a particle size D₅₀ for the slurry in a range from 0.5 to 5         mm, for example in a range from 750 μm to 3.5 mm, particularly         preferably in a range from 1 to 2.5 mm;     -   VIII/a metal content of metals which are different to aluminium,         of less than 2000 ppb, for example of less than 500 ppb,         particularly preferably of less than 100 ppb;     -   IX/viscosity (p=1013 hPa) in a range from log₁₀ (η(1250°         C.)/dPas)=11.4 to log₁₀ (η(1250° C.)/dPas)=12.9 and/or log₁₀         (η(1300° C.)/dPas)=11.1 to log₁₀ (η(1300° C.)/dPas)=12.2 and/or         log₁₀ (η(1350° C.)/dPas)=10.5 to log₁₀ (η(1350° C.)/dPas)=11.5;     -   wherein the ppb and ppm are each based on the total weight of         the quartz glass grain.

Preferably, the quartz glass grain has a metal content of metals different to aluminium of less than 1000 ppb, for example of less than 500 ppb, particularly preferably of less than 100 ppb, in each case based on the total weight of the quartz glass grain. Often however, the quartz glass grain has a content of metals different to aluminium of at least 1 ppb. Such metals are for example sodium, lithium, potassium, magnesium, calcium, strontium, germanium, copper, molybdenum, titanium, iron and chromium. These can for example be present as an element, as part of a molecule or of an ion or of a complex.

The quartz glass grain can comprise further constituents. Preferably, the quartz glass grain comprises less than 500 ppm, for example less than 450 ppm, particularly preferably less than 400 ppm of further constituents, wherein the ppm are each based on the total weight of the quartz glass grain. Often however, the quartz glass grain has a carbon content of at least 1 ppb.

Preferably, the quartz glass grain comprises carbon in a quantity of less than 5 ppm, for example less than 4 ppm or less than 3 ppm, particularly preferably less than 2 ppm, in each case based on the total weight of the quartz glass grain. Often however, the quartz glass grain has a carbon content of at least 1 ppb.

The quartz glass grain preferably has the feature combination I//II//III/ or II/II//IV/ or I//II//V/, further preferred the feature combination I//II//III/IV/ or I//II//III/V/ or I//II//IV/V/, further preferably the feature combination I//II//III/IV/V/.

The quartz glass grain preferably has the feature combination I//II//III/, wherein the OH content is less than 400 ppm, the chlorine content is less than 40 ppm and the aluminium content is less than 200 ppb.

The quartz glass grain preferably has the feature combination I//II//IV/, the OH content is less than 400 ppm, the chlorine content is less than 40 ppm and the BET surface area is less than 0.5 m²/g.

The quartz glass grain preferably has the feature combination I//II//V/, wherein the OH content is less than 400 ppm, the chlorine content is less than 40 ppm and the bulk density is in a range from 1.15 to 1.35 g/cm³.

The quartz glass grain preferably has the feature combination wherein the OH content is less than 400 ppm, the chlorine content is less than 40 ppm, the aluminium content is less than 200 ppb and the BET surface area is less than 0.5 m²/g.

The quartz glass grain preferably has the feature combination I//II//III/V/, wherein the OH content is less than 400 ppm, the chlorine content is less than 40 ppm, the aluminium content is less than 200 ppb and the bulk density is in a range from 1.15 to 1.35 g/cm³.

The quartz glass grain preferably has the feature combination I//II//IV/V/, wherein the OH content is less than 400 ppm, the chlorine content is less than 40 ppm, the BET surface area is less than 0.5 m²/g and the bulk density is in a range from 1.15 to 1.35 g/cm³.

The quartz glass grain preferably has the feature combination I//II//III/IV/V/, wherein the OH content is less than 400 ppm, the chlorine content is less than 40 ppm, the aluminium content is less than 200 ppb, the BET surface area is less than 0.5 m²/g and the bulk density is in a range from 1.15 to 1.35 g/cm³.

A second aspect of the present invention is a quartz glass grain obtainable by the process according to the first aspect of the invention.

Preferably, the quartz glass grain has at least one of the following features, for example at least two or at least three or at least four, particularly preferably at least five of the following features:

-   -   I/an OH content of less than 500 ppm, for example of less than         400 ppm, particularly preferably of less than 300 ppm;     -   II/a chlorine content of less than 60 ppm, preferably of less         than 40 ppm, for example of less than 40 ppm or less than 2 ppm         or less than 0.5 ppm, particularly preferably of less than 0.1         ppm;     -   III/an aluminium content of less than 1000 ppb, for example of         less than 500 ppb, particularly preferably of less than 200 ppb;     -   IV/a BET surface area of less than 1 m²/g, for example of less         than 0.5 m²/g, particularly preferably of less than 0.2 m²/g;     -   V/a bulk density in a range from 1.1 to 1.4 g/cm³, for example         in a range from 1.15 to 1.35 g/cm³, particularly preferably in a         range from 1.2 to 1.3 g/cm³;     -   VII a particle size D₅₀ for melting in a range from 50 to 500         μm, for example in a range from 75 to 350 μm, particularly         preferably in a range from 100 to 250 μm;     -   VII/a particle size D₅₀ for the slurry in a range from 0.5 to 5         mm, for example in a range from 750 μm to 3.5 mm, particularly         preferably in a range from 1 to 2.5 mm;     -   VIII/a metal content of metals which are different to aluminium,         of less than 2000 ppb, for example of less than 500 ppb,         particularly preferably of less than 100 ppb;     -   IX/viscosity (p=1013 hPa) in a range from log₁₀ (η(1250°         C.)/dPas)=11.4 to log₁₀ (η(1250° C.)/dPas)=12.9 and/or log₁₀         (η(1300° C.)/dPas)=11.1 to log₁₀ (η(1300° C.)/dPas)=12.2 and/or         log₁₀ (1350° C.)/dPas)=10.5 to log₁₀ (η(1350° C.)/dPas)=11.5;     -   wherein the ppb and ppm are each based on the total weight of         the quartz glass grain.

A third aspect of the present invention is a process for the preparation of an opaque formed body. The process comprises the following steps:

-   -   (1) Providing a quartz glass grain according to the second         aspect; and     -   (2) Processing the quartz glass grain to an opaque formed body.

Step (1)

The quartz glass grain prepared in step (1) is preferably characterised by the features described in the context of the second aspect of the invention. The quartz glass grain provided according to step (1) is obtainable by a process according to the first aspect of the invention.

Step (2)

It is in principle possible to process the quartz glass grain to an opaque formed body in all ways which are known to the skilled man and suitable for this purpose.

Preferably, the processing of the quartz glass grain provided in step (1) to an opaque formed body comprises the following steps:

-   -   v.) Processing the quartz glass grain to a preform; and     -   vi.) Making an opaque formed body from the preform obtained in         step v.).

Step v.)

To process the quartz glass grain to a preform, in principle all ways which are known to the skilled man and suitable for forming a preform from a grain are possible. For example, the preform can be made from a dry quartz glass grain or from a slurry of a quartz glass grain.

According to a preferred embodiment, making the preform comprises the following individual steps:

-   -   v.-1.) Making a slurry of the quartz glass grain with a liquid;         and     -   v.-2.) Forming a preform from the slurry by removing liquid.

According to step v.-1.), firstly a slurry of the quartz glass grain is made with a liquid. In what follows this is referred to as a “slurry comprising a quartz glass grain”.

In principle, suitable liquids are all materials and mixtures of materials known to the skilled man and which appear suitable for the present application. Preferably, the liquid is selected from the group consisting of organic liquids and water.

Preferred suitable liquids are polar solvents. These can be organic liquids or water. Preferably, the liquid is selected from the group consisting of water, methanol, ethanol, n-propanol, iso-propanol, n-butanol, tert-butanol and mixtures of more than one thereof. Particularly preferably, the liquid is water. Particularly preferably, the liquid comprises distilled or de-ionised water.

The quartz glass grain and the liquid can be mixed in any manner For example, the quartz glass grain can be added to the liquid, or the liquid can be added to the quartz glass grain. The mixture can be agitated during the addition or following the addition. Particularly preferably, the mixture is agitated during and following the addition. Examples for the agitation are shaking and stirring, or a combination of both. Preferably, the quartz glass grain can be added to the liquid under stirring. Furthermore, preferably, a portion of the quartz glass grain can be added to the liquid, wherein the mixture thus obtained is agitated, and the mixture is subsequently mixed with the remaining portion of the quartz glass grain. Likewise, a portion of the liquid can be added to the quartz glass grain, wherein the mixture thus obtained is agitated, and the mixture subsequently mixed with the remaining portion of the liquid.

Preferably, the slurry comprising quartz glass grain has a weight per litre in the range from 700 to 2000 g/L, for example in the range from 1000 to 1900 g/L or from 1200 to 1800 g/L, particularly preferably in the range from 1400 to 1700 g/L. The weight per litre is measured by weighing a volume calibrated container Preferably, the solids content in the liquid is at least 70%, for example at least 75%, more preferably from 75 to 80%, based on the total weight of the liquid. Often, a solids content of 85% is not exceeded, based on the total weight of the liquid.

The quartz glass grain introduced into the liquid has preferably a particle size D₅₀ in a range from 50 μm to 5 mm.

A slurry comprising a quartz glass grain with a particle size D₅₀ of more than 500 μm is subjected to a grinding process preferably before further processing according to step v.-2.).

A slurry comprising a quartz glass grain with a particle size D₅₀ in a range from 50 to 500 μm can be further processed directly according to step v.-2.).

Preferably, a slurry comprising a quartz glass grain with a particle size D₅₀ in a range from 50 to 500 μm is subjected to a grinding process before further processing according to step v.-2.). The slurry obtained subsequent to a grinding process comprises preferably a quartz glass grain with a particle size D₅₀ in a range from 10 to 60 μm. In principle, all grinding processes known to the skilled man and suitable for this purpose can be carried out on the slurry.

According to step v.-2.), a preform is formed from the slurry comprising a quartz glass grain by removing liquid.

Preferably, the preform is formed in a container, preferably in a container made from a material that is permeable to water, particularly preferably in a container made from plaster of Paris, or from plastic. Preferably, the slurry is drained in the container to form of a green body. The green body is preferably removed from the container, dried, processed and sintered. Draining takes place preferably by suction, pressing or centrifuging, wherein the liquid preferably passes through the material of the container.

Preferably, the green body is charged with a pressure which is reduced compared with normal pressure, preferably in a range from 0.001 to 0.9 bar, for example in a range from 0.01 to 0.5 bar, particularly preferably in a range from 0.05 to 0.2 bar.

Preferably, the green body is dried by a die cast process, wherein the liquid is pressed out of the green body via a pressure gradient. Preferably, for this, an overpressure is applied from above or an underpressure is applied from below, or both. Due to an overpressure from above, the green body and the liquid located therein is pressed downwards into the mould, wherein the liquid is pressed through the mould and thus is removed from the green body. An overpressure is employed to mean a pressure which is higher than normal pressure. The overpressure is preferably in a range from 1.2 to 10 bar, particularly preferably in a range from 1.2 to 2.5 bar. In the case of an underpressure from below, a reduced pressure is built up underneath the mould, from the outside. An underpressure is employed to mean a pressure which is reduced compared with normal pressure. Due to the underpressure, liquid is sucked downwards through the mould and thereby removed from the green body. Preferably, an underpressure in a range from 0.001 to 0.9 bar for example in a range from 0.01 to 0.5 bar, particularly preferably in a range from 0.05 to 0.2 bar, is applied.

Preferably, the green body is dried after being removed from the container, for example by heating, for example to a temperature in a range from 30 to 700° C. or from 30 to 100° C., particularly preferably from 40 to 90° C.

Preferably, the green body is tempered at the above described temperature for a period in a range from 4 h to 14 days, for example for a period in a range from 10 h to 10 days or from 1 day to 7 days, particularly preferably from 3 to 5 days.

Preferably, the green body is dried for as long as it takes for the liquid to be removed. Thus a liquid-poor or even liquid-free preform is formed. Preferably, the preform has a content of liquid, in particular water, of less than 5 wt.-%, for example less than 2 wt.-%, particularly preferably less than 1 wt.-%, the weight percent in each case based on the total weight of the preform. Often, a content of liquid in the preform of 0.01 wt.-% is not undershot, the weight percent being based on the total weight of the preform.

According to another preferred embodiment, the making of the preform comprises the following step:

-   -   v.-a.) introducing the quartz glass grain into a rotating hollow         mould.

Preferably, due to the introeduction into the rotating hollow mould, the quartz glass grain is stored as a sheet on the inner wall of the rotating hollow mould.

The rotating hollow mould can be pot-shaped, i.e. can have a floor. The hollow mould can be solid or gas-permeable. Preferably it has bores. Further preferably, an underpressure can be applied to the hollow mould from the outside. Thus the filled-in quartz glass grain can be fixed in the hollow mould. According to a further preferred embodiment, the grain can be endowed with an inner contour during rotation, for example by applying a master plate.

According to an alternative embodiment of step v.-a.), the quartz glass grain is introduced into a tubular hollow mould. This has preferably a horizontal axis of rotation. The hollow mould can be solid or gas-permeable. Preferably it has bores. Further preferably, a vacuum is applied, with which the filled-in quartz glass grain is fixed to the hollow mould. According to a further preferred embodiment, the grain can be endowed with an inner contour during rotation, for example by applying a master plate.

Step vi.)

According to a preferred embodiment of the first aspect of the present invention, the process comprises, as step vi.), the formation of the opaque quartz glass body from the preform obtained in step v.).

In principle, the opaque quartz glass body can be made in any ways known to the skilled man and which appear suitable for same, in particular, preferably by heating the preform.

According to a preferred embodiment, the preform can be heated in an oven. For example, the heating comprises a sintering. Sintering is employed to mean heating at a specific pressure to a temperature below the melting range of a material present as a grain, in which the grains are shaped to form a sintered body. In respect of the quartz glass grain according to the invention, a sintering can be carried out for example at a pressure of 1 bar and a temperature in a range from 1350 to 1450° C. In the sintering, the grain boundaries between the grains which have been formed to obtain the sintered body are retained. The optical opacity results from a deflection of incident light at these grain boundaries. Further preferably, the forming of the opaque quartz glass body in step vi.) is a sintering.

Preferably a preform which is obtained according to steps v.-1.) and v.-2.) is heated in an oven.

Preferably, the preform is heated to a temperature of more than 1100° C., for example of more than 1200 or of more than 1300 or of more than 1350° C. Preferably, the preform is heated to a temperature of less than 1700° C. Preferably, the preform is heated to a temperature in a range from 1100 to 1700° C., for example in a range from 1200 to 1600° C. or from 1300 to 1500° C., particularly preferably in a range from 1350 to 1450° C.

Preferably, the preform is heated for a period of at least 40 min, for example for a period in a range from 1 to 240 h or from 2 to 150 h or from 5 to 100 h, preferably from 10 to 50 h.

Preferably, the preform is heated in a gas atmosphere, for example in an inert gas atmosphere, in an oxidising atmosphere or in a reducing atmosphere, particularly preferably in an oxidising atmosphere. An inert gas atmosphere comprises preferably exclusively inert gases, for example nitrogen, helium, neon, argon, krypton and xenon or combinations of two or more thereof, particularly preferably nitrogen, argon or a combination thereof. An oxidising atmosphere comprises preferably oxygen, for example a combination of oxygen and one or more than one inert gas, particularly preferably air. A reducing atmosphere comprises preferably hydrogen, for example a combination of hydrogen and one or more than one inert gas, particularly preferably a combination of hydrogen and helium. Particularly preferably the atmosphere during sintering is air.

According to a further preferred embodiment, the preform can be heated by a plasma, preferably one or more electric arcs. Preferably, a preform which is obtained according to step v.-a.) is heated by a plasma, preferably by an electric arc.

According to a further preferred embodiment, a vacuum can be applied, from the outside, to the rotating hollow mould with bores also when making the quartz glass body. Thus the bubble content of the melted material can be adjusted.

According to a further preferred embodiment, the interior of the rotating hollow mould can be charged with an underpressure. A plasma electric arc is ignited along the axis of rotation between two or more electrodes, which electric arc melts the synthetic grain to a transparent or opaque glass in the form of a cylinder. The process can be repeated iteratively in order to build up a plurality of sheets into layers and increase the strength of the wall.

Preferably, the rotating hollow mould comprises a gas inlet, for example made of quartz glass. A gas can be introduced into the rotating hollow mould through the gas inlet.

Preferably, the opaque quartz glass body obtained in this way is cooled, preferably at a cooling rate of more than 5 K/min, for example in a range from 10 to 60 K/min, particularly preferably in a range from 15 to 30 K/min.

The quartz glass bodies obtainable according to the first aspect of the invention can in principle have any shape. For example, opaque quartz glass bodies can be made in solid form, such as cylinders, sheets, plates, cuboids, spheres, or in the form of hollow bodies, e.g. hollow bodies with an opening, hollow bodies with two openings such as tubes and flanges, or with more than two openings. By using a rotating hollow mould, obtainable opaque quartz glass bodies have preferably a rotation-symmetrical, hollow shape with two openings, for example the shape of tubes or flanges. By using a container, opaque quartz glass bodies are preferably solid.

A sheet is employed to mean a planar extent of a material on a substrate.

Preferably, the opaque quartz glass body has the following features:

-   -   [I] an opacity of more than 5, for example more than 10 or more         than 20 or more than 50;     -   [II] a BET surface area of less than 1 m²/g, for example of less         than 0.5 m²/g, particularly preferably of less than 0.2 m²/g;

The opacity (“O”) is a measure of the light resistance of a material. It corresponds to the ratio of the intensity of the incident light I₀ to the intensity of the emergent light I and thus the reciprocal value of the transmission of a body (cf. formula (I)).

O=I ₀ /I   (I)

The term “opaque” is employed to mean a material with an opacity (I₀/I) of than 2 at wavelengths in the range from 400 to 780 nm and with a sheet thickness of the material sample of 10 mm. Preferably, the opaque quartz glass body has an opacity of more than 5, for example of more than 10 or of more than 20 or of more than 50.

Preferably, the opaque quartz glass body additionally has at least one, for example at least two or at least three or at least four, particularly preferably at least five of the following features:

-   -   [III] a cylindrical form;     -   [IV] a layer;     -   [V] an OH content of less than 500 ppm, for example of less than         400 ppm, particularly preferably of less than 300 ppm;     -   [VI] a chlorine content of less than 60 ppm, preferably of less         than 40 ppm, for example of less than 40 ppm or less than 2 ppm         or less than 0.5 ppm, particularly preferably of less than 0.1         ppm;     -   [VII] an aluminium content of less than 200 ppb, for example of         less than 150 ppb, particularly preferably of less than 100 ppb;     -   [VIII] an ODC content of less than 5*10¹⁸/cm³;     -   [IX] a carbon content of less than 5 ppm, for example of less         than 4 ppm or of less than 3 ppm, particularly preferably of         less than 2 ppm;     -   [X] a total metal content of metals different to aluminium of         less than 2000 ppb, for example of less than 500 ppb,         particularly preferably of less than 100 pbb;     -   wherein the ppm and ppb are each based on the total weight of         the opaque quartz glass body.

Preferably, the opaque quartz glass body has a metal content of metals different to aluminium of less than 1000 ppb, for example of less than 500 ppb, particularly preferably of less than 100 ppb, in each case based on the total weight of the opaque quartz glass body. Often however, the opaque quartz glass body has a content of metals different to aluminium of at least 1 ppb. Such metals are for example sodium, lithium, potassium, magnesium, calcium, strontium, germanium, copper, molybdenum, titanium, iron and chromium. These can be present, for example, as an element, as part of a molecule or of an ion or of a complex.

The opaque quartz glass body can comprise further constituents. Preferably, the opaque quartz glass body comprises less than 500 ppm, for example less than 450 ppm, particularly preferably less than 400 ppm of further constituents, the ppm in each case based on the total weight of the opaque quartz glass body. Often however, the opaque quartz glass body has a content of further constituents of at least 1 ppb.

Preferably, the opaque quartz glass body comprises carbon in a quantity of less than 5 ppm, for example less than 4 ppm or less than 3 ppm, particularly preferably less than 2 ppm, in each case based on the total weight of the opaque quartz glass body. Often however, the opaque quartz glass body has a carbon content of at least 1 ppb.

The opaque quartz glass body preferably has the feature combination [I]/[II]/[VI] or [I]/[II]/[VIII] or [I]/[II]/[IX], further preferred the feature combination [I]/[II]/[VI]/[VIII] or [I]/[II]/[VI]/[IX] or [I]/[II]/[VIII]/[IX], further preferably the feature combination [I]/[II]/[VI]/[VIII]/[IX].

The opaque quartz glass body preferably has the feature combination [I]/[II]/[VI], wherein the opacity is more than 20, the BET surface area is less than 0.2 m²/g and the chlorine content is less than 20 ppm.

The opaque quartz glass body preferably has the feature combination [I]/[II]/[VIII], wherein the opacity is more than 20, the BET surface area is less than 0.2 m²/g and the ODC content is less than 5*10¹⁸/cm³.

The opaque quartz glass body preferably has the feature combination [I]/[II]/[IX], wherein the opacity is more than 20, the BET surface area is less than 0.2 m²/g and the carbon content is less than 3 ppm.

The opaque quartz glass body preferably has the feature combination [I]/[II]/[VI]/[VIII], wherein the opacity is more than 20, the BET surface area is less than 0.2 m²/g, the chlorine content is less than 20 ppm and the ODC content is less than 5*10¹⁸/cm³.

The opaque quartz glass body preferably has the feature combination [I]/[II]/[VI]/[IX], wherein the opacity is more than 20, the BET surface area is less than 0.2 m²/g the chlorine content is less than 20 ppm and the carbon content is less than 3 ppm.

The opaque quartz glass body preferably has the feature combination [I]/[II]/[VIII]/[IX], wherein the opacity is more than 20, the BET surface area is less than 0.2 m²/g, the ODC content is less than 5*10¹⁸/cm³ and the carbon content is less than 3 ppm.

The opaque quartz glass body preferably has the feature combination [I]/[III]/[VI]/[VIII]/[IX], wherein the opacity is more than 20, the BET surface area is less than 0.2 m²/g the chlorine content is less than 20 ppm, the ODC content is less than 5*10¹⁸/cm³ and the carbon content is less than 3 ppm.

Preferred opaque formed bodies are, for example

-   -   opaque rings, for example opaque rings which are used in         conjunction with tubes (spacers) or which are used in         conjunction with transparent plates (domes);     -   opaque flanges, for example opaque flanges which are used in         conjunction with tubes;     -   opaque plates, for example opaque plates which are used in the         foundation of boats in equipment for preparing semiconductors         (pedestals), in particular for the targeted thermal screening of         a boat from the pedestal;     -   opaque linings of single-wafer systems (liners);     -   domes for epitaxy systems or rapid thermal processing (RTP)         systems.

A fourth aspect of the present invention is an opaque formed body obtainable by a process comprising the following steps:

-   -   (1) Provision of a quartz glass grain; and     -   (2) Processing the quartz glass grain to an opaque formed body.

The steps (1) and (2) are preferably characterised by the features described in the context of the third aspect of the invention.

The opaque formed body is preferably characterised by the features described in the context of the third aspect.

A fifth aspect of the present invention is the use of a quartz glass grain for the preparation of opaque quartz glass and of products comprising opaque quartz glass, wherein the quartz glass grain has the following features:

-   -   I/an OH content of less than 500 ppm;     -   II/a chlorine content of less than 60 ppm;     -   III/an aluminium content of less than 200 ppb.     -   wherein the ppb and ppm are each based on the total weight of         the quartz glass grain.

FIGURES

FIG. 1 flow diagram (process for the preparation of a quartz glass body)

FIG. 2 flow diagram (process for the preparation of a silicon dioxide granulate I)

FIG. 3 flow diagram (process for the preparation of a silicon dioxide granulate II)

FIG. 4 flow diagram (process for the preparation of a quartz glass grain)

FIG. 5 flow diagram (process for the preparation of an opaque quartz glass body)

FIG. 6 schematic representation of a hanging crucible in an oven

FIG. 7 schematic representation of a standing crucible in an oven

FIG. 8 schematic representation of a crucible with a flushing ring

FIG. 9 schematic representation of a spray tower

FIG. 10 schematic representation of a crucible with a dew point measuring device

FIG. 11 schematic representation of a gas pressure sinter oven (GDS oven)

DESCRIPTION OF THE FIGURES

FIG. 1 shows a flow diagram containing the steps 101 to 104 of a process 100 for the preparation of a quartz glass body according to the present invention. In a first step 101, a silicon dioxide granulate is provided. In a second step 102, a glass melt is made from the silicon dioxide granulate.

Preferably, moulds are used for the melting which can be introduced into and removed from an oven. Such moulds are often made of graphite. They provide a negative form for the caste item. The silicon dioxide granulate is filled into the mould and is first melted in the mould in step 103. Subsequently, the quartz glass body is formed in the same mould by cooling the melt. It is then freed from the mould and processed further, for example in an optional step 104. This procedure is discontinuous. The forming of the melt is preferably performed at reduced pressure, in particular in a vacuum. Further, it is possible during step 103 to charge the oven intermittently with a reducing, hydrogen containing atmosphere.

In another procedure, hanging or standing crucibles are preferably employed. The melting is preferably performed in a reducing, hydrogen containing atmosphere. In a third step 103, a quartz glass body is formed. The formation of the quartz glass body is preferably performed by removing at least a part of the glass melt from the crucible and cooling. The removal is preferably performed through a nozzle at the lower end of the crucible. In this case, the form of the quartz glass body can be determined by the design of the nozzle. In this way, for example, solid bodies can be obtained. Hollow bodies are obtained for example if the nozzle additionally has a mandrel. This example of a process for the preparation of quartz glass bodies, and in particular step 103, is preferably performed continuously. In an optional step 104, a hollow body can be formed from a solid quartz glass body.

FIG. 2 shows a flow diagram containing the steps 201, 202 and 203 of a process 200 for the preparation of a silicon dioxide granulate I. In a first step 201, a silicon dioxide powder is provided. A silicon dioxide powder is preferably obtained from a synthetic process in which a silicon containing material, for example a siloxane, a silicon alkoxide or a silicon halide is converted into silicon dioxide in a pyrogenic process. In a second step 202, the silicon dioxide powder is mixed with a liquid, preferably with water, to obtain a slurry. In a third step 203, the silicon dioxide contained in the slurry is transformed into a silicon dioxide granulate. The granulation is performed by spray granulation For this, the slurry is sprayed through a nozzle into a spray tower and dried to obtain granules, wherein the contact surface between the nozzle and the slurry comprises a glass or a plastic.

FIG. 3 shows a flow diagram containing the steps 301, 302, 303 and 304 of a process 300 for the preparation of a silicon dioxide granulate II. The steps 301, 302 and 303 proceed corresponding to the steps 201, 202 and 203 according to FIG. 2. In step 304, the silicon dioxide granulate I obtained in step 303 is processed to obtain a silicon dioxide granulate II. This is preferably performed by warming the silicon dioxide granulate I in a chlorine containing atmosphere.

FIG. 4 shows a flow diagram containing the steps 601 to 604 of a process for the preparation of a quartz glass grain 600. In a first step 601, a silicon dioxide granulate is provided. In a second step 602, a glass melt is formed from the silicon dioxide. Preferably, for this, the silicon dioxide granulate is introduced into a melting crucible and heated therein until a glass melt forms. Preferably, hanging metal sheet crucibles or sintering crucibles or standing sintering crucibles are used as melting crucibles. Melting takes place preferably in a reducing atmosphere containing hydrogen. In a third step 603 a quartz glass body is made. The quartz glass body is made by removing at least one part of the glass melt from the crucible and cooling. Removal takes place preferably by a nozzle at the bottom end of the crucible. The shape of the quartz glass body can be determined by the design of the nozzle. Preparation of quartz glass bodies, and in particular step 603, takes place preferably continuously. In a fourth step 604 the size of the quartz glass body is reduced, preferably by high voltage discharge pulses to obtain a quartz glass grain.

FIG. 5 shows a flow diagram containing the steps 701 to 706 of a process for the preparation of an opaque quartz glass grain 700. In a first step 701, a silicon dioxide granulate is provided. In a second step 702, a glass melt is formed from the silicon dioxide. Preferably, for this, the silicon dioxide granulate is introduced into a melting crucible and heated therein until a glass melt forms. Preferably, hanging metal sheet crucibles or sintering crucibles or standing sintering crucibles are used as melting crucibles. Melting takes place preferably in a reducing atmosphere containing hydrogen. In a third step 703 a quartz glass body is made. The quartz glass body is made by removing at least one part of the glass melt from the crucible and cooling. Removal takes place preferably by a nozzle at the bottom end of the crucible. The shape of the quartz glass body can be determined by the design of the nozzle. Preparation of quartz glass bodies, and in particular step 703, takes place preferably continuously. In a fourth step 704 the size of the quartz glass body is reduced, preferably by high voltage discharge pulses to obtain a quartz glass grain. In a fifth step 705 the quartz glass grain is processed into a preform. For this, the quartz glass grain is preferably mixed with a liquid to obtain a slurry, and shaped to make a preform, by removing liquid. Shaping takes place preferably by introducing the slurry into a container, wherein the shape of the preform is established by the design of the container. Alternatively, the preform is made by introducing the quartz glass grain into a rotating hollow mould. In a sixth step 706, the preform is heated to obtain an opaque quartz glass body.

In FIG. 6, a preferred embodiment of an oven 800 with a hanging crucible is shown. The crucible 801 is arranged hanging in the oven 800. The crucible 801 has a hanger assembly 802 in its upper region, as well as a solids inlet 803 and a nozzle 804 as outlet. The crucible 801 is filled via the solids inlet 803 with silicon dioxide granulate 805. In operation, silicon dioxide granulate 805 is present in the upper region of the crucible 801, whilst a glass melt 806 is present in the lower region of the crucible. The crucible 801 can be heated by heating elements 807 which are arranged on the outer side of the crucible wall 810. The oven also has an insulation layer 809 between the heating elements 807 and the outer wall 808 of the oven. The space in between the insulation layer 809 and the crucible wall 810 can be filled with a gas and for this purpose has a gas inlet 811 and a gas outlet 812. A quartz glass body 813 can be removed from the oven through the nozzle 804.

In FIG. 7 a preferred embodiment of an oven 900 with a standing crucible is shown. The crucible 901 is arranged standing in the oven 900. The crucible 901 has a standing area 902, a solids inlet 903 and a nozzle 904 as outlet. The crucible 901 is filled with silicon dioxide granulate 905 via the inlet 903. In operation, silicon dioxide granulate 905 is present in the upper region of the crucible, whilst a glass melt 906 is present in the lower region of the crucible. The crucible can be heated by heating elements 907 which are arranged on the outer side of the crucible wall 910. The oven also has an insulation layer 909 between the heating elements 907 and the outer wall 908. The space between the insulation layer 909 and the crucible wall 910 can be filled with a gas and for this purpose has a gas inlet 911 and a gas outlet 912. A quartz glass body 913 can be removed from the crucible 901 through the nozzle 904.

In FIG. 8 is shown a preferred embodiment of a crucible 1000. The crucible 1000 has a solids inlet 1001 and a nozzle 1002 as outlet. The crucible 1000 is filled with silicon dioxide granulate 1003 via the solids inlet 1001. In operation, silicon dioxide granulate 1003 is present as a reposing cone 1004 in the upper region of the crucible 1000, whilst a glass melt 1005 is present in the lower region of the crucible. The crucible 1000 can be filled with a gas. It has a gas inlet 1006 and a gas outlet 1007. The gas inlet is a flushing ring mounted on the crucible wall above the silicon dioxide granulate. The gas in the interior of the crucible is released through the flushing ring (with a gas feed not shown here) close above the melting level and/or the reposing cone near the crucible wall and flows in the direction of the gas outlet 1007 which is arranged as a ring in the lid 1008 of the crucible 1000. The gas flow 1010 which is produced in this way moves along the crucible wall and submerges it. A quartz glass body 1009 can be removed from the crucible 1000 through the nozzle 1002.

In FIG. 9 is shown a preferred embodiment of a spray tower 1100 for spray granulating silicon dioxide. The spray tower 1100 comprises a feed 1101 through which a pressurised slurry containing silicon dioxide powder and a liquid are fed into the spray tower. At the end of the pipeline is a nozzle 1102 through which the slurry is introduced into the spray tower as a finely spread distribution. Preferably, the nozzle slopes upward, so that the slurry is sprayed into the spray tower as fine droplets in the nozzle direction and then falls down in an arc under the influence of gravity. At the upper end of the spray tower there is a gas inlet 1103. By introduction of a gas through the gas inlet 1103, a gas flow is created in the opposite direction to the exit direction of the slurry out of the nozzle 1102. The spray tower 1100 also comprises a screening device 1104 and a sieving device 1105. Particles which are smaller than a defined particle size are extracted by the screening device 1104 and removed through the discharge 1106. The extraction strength of the screening device 1104 can be configured to correspond to the particle size of the particles to be extracted. Particles above a defined particle size are sieved off by the sieving device 1105 and removed through the discharge 1107. The sieve permeability of the sieving device 1105 can be selected to correspond to the particle size to be sieved off. The remaining particles, a silicon dioxide granulate having the desired particle size, are removed through the outlet 1108.

FIG. 10 shows a preferred embodiment of a crucible 1400. The crucible has a solids inlet 1401 and an outlet 1402. In operation, silicon dioxide granulate 1403 is present in a reposing cone 1404 in the upper region of the crucible 1400, whilst a glass melt 1405 is present in the lower region of the crucible. The crucible 1400 has a gas inlet 1406 and a gas outlet 1407. The gas inlet 1406 and the gas outlet 1407 are arranged above the reposing cone 1404 of the silicon dioxide granulate 1403. The gas outlet 1406 comprises a pipeline for the gas feed 1408 and a device 1409 for measuring the dew point of the exiting gas. The device 1409 comprises a dew point mirror hygrometer (not shown here). The separation between the crucible and the device 1409 for measuring the dew point can vary. A quartz glass body 1410 can be removed through the outlet 1402 of the crucible 1400.

FIG. 11 shows a preferred embodiment of the oven 1500 which is suitable for a vacuum sintering process, a gas pressure sinter process and in particular a combination thereof. The oven has from outside inward a pressure resistant jacket 1501 and a thermal insulating layer 1502. The space enclosed thereby, referred to as the oven interior, can be charged with a gas or a gas mixture via a gas feed 1504. Further, the oven interior has a gas outlet 1505 via which gas can be removed. According to the gas transport balance between gas feed 1504 and gas removal at 1505 an overpressure, a vacuum or also a gas flow can be produced in the interior of the oven 1500. Further, heating elements 1506 are present in the oven interior 1500. These are often mounted on the insulation layer 1502 (not shown here). For protecting the melt material from contamination, there is a so-called “liner” 1507 in the interior of the oven, which separates the oven chamber 1503 from the heating elements 1506. Moulds 1508 with material to be melted 1509 can be introduced into the oven chamber 1503. The mould 1508 can be open on a side (shown here) or can completely enclose the melt material 1509 (not shown).

Test Methods

a. Fictive Temperature

The fictive temperature is measured by Raman spectroscopy using the Raman scattering intensity at about 606 cm⁻¹. The procedure and analysis described in the contribution of Pfleiderer et. al.; “The UV-induced 210 nm absorption band in fused Silica with different thermal history and stoichiometry”; Journal of Non-Crystalline Solids, volume 159 (1993), pages 145-153.

b. OH Content

The OH content of the glass is measured by infrared spectroscopy. The method of D. M. Dodd & D. M. Fraser “Optical Determinations of OH in Fused Silica” (J.A.P. 37, 3991 (1966)) is employed. Instead of the device named therein, an FTIR-spectrometer (Fourier transform infrared spectrometer, current System 2000 of Perkin Elmer) is employed. The analysis of the spectra can in principle be performed on either the absorption band at ca. 3670 cm⁻¹ or on the absorption band at ca. 7200 cm⁻¹. The selection of the band is made on the basis that the transmission loss through OH absorption is between 10 and 90%.

c. Oxygen Deficiency Centers (ODCs)

For the quantitative detection, the ODC(I) absorption is measured at 165 nm by means of a transmission measurement at a probe with thickness between 1-2 mm using a vacuum UV spectrometer, model VUVAS 2000, of McPherson, Inc. (USA).

Then:

N=α/σ

with

-   -   N=defect concentration [1/cm³]

α=optical absorption [1/cm, base e] of the ODC(I) band

σ=effective cross section [cm²]

wherein the effective cross section is set to σ=7.5·10⁻¹⁷ cm² (from L. Skuja, “Color Centers and Their Transformations in Glassy SiO₂”, Lectures of the summer school “Photosensitivity in optical Waveguides and glasses”, Jul. 13-18, 1998, Vitznau, Switzerland).

d. Elemental Analysis

d-1) Solid samples are crushed. Then, ca. 20g of the sample is cleaned by introducing it into a HF-resistant vessel fully, covering it with HF and thermally treating at 100° C. for an hour. After cooling, the acid is discarded and the sample cleaned several times with high purity water. Then, the vessel and the sample are dried in the drying cabinet.

Next, ca. 2 g of the solid sample (crushed material cleaned as above; dusts etc. without pre-treatment) is weighed into an HF resistant extraction vessel and dissolved in 15 ml HF (50 wt.-%). The extraction vessel is closed and thermally treated at 100° C. until the sample is completely dissolved. Then, the extraction vessel is opened and further thermally treated at 100° C., until the solution is completely evaporated. Meanwhile, the extraction vessel is filled 3× with 15 ml of high purity water. lml HNO₃ is introduced into the extraction vessel, in order to dissolve separated impurities and filled up to 15 ml with high purity water. The sample solution is then ready.

d-2) ICP-MS /ICP-OES Measurement

Whether OES or MS is employed depends on the expected elemental concentrations. Typically, measurements of MS are 1 ppb, and for OES they are lOppb (in each case based on the weighed sample). The measurement of the elemental concentration with the measuring device is performed according to the stipulations of the device manufacturer (ICP-MS: Agilent 7500ce; ICP-OES: Perkin Elmer 7300 DV) and using certified reference liquids for calibration. The elemental concentrations in the solution (15 ml) measured by the device are then converted based on the original weight of the probe (2 g).

Note: It is to be kept in mind that the acid, the vessels, the water and the devices must be sufficiently pure in order to measure the elemental concentrations in question. This is checked by extracting a blank sample without quartz glass.

The following elements are measured in this way: Li, Na, Mg, K, Ca, Fe, Ni, Cr, Hf, Zr, Ti, (Ta), V, Nb, W, Mo, Al.

d-3) The measurement of samples present as a liquid is carried out as described above, wherein the sample preparation according to step d-1) is skipped. 15 ml of the liquid sample are introduced into the extraction flask. No conversion based on the original sample weight is made.

e. Determination of Density of a Liquid

For measuring the density of a liquid, a precisely defined volume of the liquid is weighed into a measuring device which is inert to the liquid and its constituents, wherein the empty weight and the filled weight of the vessel are measured. The density is given as the difference between the two weight measurements divided by the volume of the liquid introduced.

f. Fluoride Determination

15 g of a quartz glass sample is crushed and cleaned by treating in nitric acid at 70° C. The sample is then washed several times with high purity water and then dried. 2 g of the sample is weighed into a nickel crucible and covered with 10 g Na₂CO₃ and 0.5 g ZnO. The crucible is closed with a Ni-lid and roasted at 1000° C. for an hour. The nickel crucible is then filled with water and boiled up until the melt crust has dissolved entirely. The solution is transferred to a 200 ml measuring flask and filled up to 200 ml with high purity water. After sedimentation of undissolved constituents, 30 ml are taken and transferred to a 100 ml measuring flask, 0.75 ml of glacial acetic acid and 60 ml TISAB are added and filled up with high purity water. The sample solution is transferred to a 150 ml glass beaker.

The measurement of the fluoride content in the sample solution is performed by means of an ion sensitive (fluoride) electrode, suitable for the expected concentration range, and display device as stipulated by the manufacturer, here a fluoride ion selective electrode and reference electrode F-500 with R503/D connected to a pMX 3000/pH/ION from Wissenschaftlich-Technische Werkstatten GmbH. With the fluoride concentration in the solution, the dilution factor and the sample weight, the fluoride concentration in the quartz glass is calculated.

g. Determination of Chlorine (>=50 ppm)

15 g of a quartz glass sample is crushed and cleaned by treating with nitric acid at ca. 70° C. Subsequently, the sample is rinsed several times with high purity water and then dried. 2 g of the sample are then filled into a PTFE-insert for a pressure container, dissolved with 15 ml NaOH (c=10 mol/l), closed with a PTFE lid and placed in the pressure container It is closed and thermally treated at ca. 155° C. for 24 hours. After cooling, the PTFE insert is removed and the solution is transferred entirely to a 100 ml measuring flask. There, 10 ml HNO₃ (65 wt.-%) and 15 ml acetate buffer and added, allowed to cool and filled to 100 ml with high purity water. The sample solution is transferred to a 150 ml glass beaker. The sample solution has a pH value in the range between 5 and 7.

The measurement of the chloride content in the sample solution is performed by means of an ion sensitive (Chloride) electrode which is suitable for the expected concentration range, and a display device as stipulated by the manufacturer, here an electrode of type C1-500 and a reference electrode of type R-503/D attached to a pMX 3000/pH/ION from Wissenschaftlich-Technische Werkstatten GmbH.

h. Chlorine Content (<50 ppm)

Chlorine content <50 ppm up to 0.1 ppm in quartz glass is measured by neutron activation analysis (NAA). For this, 3 bores, each of 3 mm diameter and 1 cm long are taken from the quartz glass body under investigation. These are given to a research institute for analysis, in this case to the institute for nuclear chemistry of the Johannes-Gutenberg University in Mainz, Germany. In order to exclude contamination of the sample with chlorine, a thorough cleaning of the sample in an HF bath on location directly before the measurement was arranged. Each bore is measured several times. The results and the bores are then sent back by the research institute.

i. Optical Properties

The transmission of quartz glass samples is measured with the commercial grating- or FTIR-spectrometer from Perkin Elmer (Lambda 900 [190-3000 nm] or System 2000 [1000-5000 nm]). The selection is determined by the required measuring range.

For measuring the absolute transmission, the sample bodies are polished on parallel planes (surface roughness RMS<0.5 nm) and the surface is cleared off all residues by ultrasound treatment. The sample thickness is 1 cm. In the case of an expected strong transmission loss due to impurities, dopants etc., a thicker or thinner sample can be selected in order to stay within the measuring range of the device. A sample thickness (measuring length) is selected at which only slight artefacts are produced on account of the passage of the radiation through the sample and at the same time a sufficiently detectable effect is measured.

The measurement of the opacity, the sample is placed in front of an integrating sphere. The opacity is calculated using the measured transmission value T according to the formula: O=1/T=I₀/I.

j. Refractive Index and Distribution of Refractive Index in a Tube or Rod

The distribution of refractive index of tubes /rods can be characterised by means of a York Technology Ltd. Preform Profiler P102 or P104. For this, the rod is placed lying in the measuring chamber the chamber is closed tight. The measuring chamber is then filled with an immersion oil which has a refractive index at the test wavelength of 633 nm, which is very similar to that of the outermost glass layer at 633 nm. The laser beam then goes through the measuring chamber. Behind the measuring chamber (in the direction of the of the radiation) is mounted a detector which measures the angle of deviation (of the radiation entering the measuring chamber compared to the radiation exiting the measuring chamber). Under the assumption of radial symmetry of the distribution of refractive index of the rod, the diametral distribution of refractive index can be reconstructed by means of an inverse Abel transformation. These calculations are performed by the software of the device manufacturer York.

The refractive index of a sample is measured with the York Technology Ltd. Preform Profiler P104 analogue to the above description. In the case of isotropic samples, measurement of distribution of refractive index gives only one value, the refractive index.

k. Carbon Content

The quantitative measurement of the surface carbon content of silicon dioxide granulate and silicon dioxide powder is performed with a carbon analyser RC612 from Leco Corporation, USA, by the complete oxidation of all surface carbon contamination (apart from SiC) with oxygen to obtain carbon dioxide. For this, 4.0 g of a sample are weighed and introduced into the carbon analyser in a quartz glass dish. The sample is bathed in pure oxygen and heated for 180 seconds to 900° C. The CO₂ which forms is measured by the infrared detector of the carbon analyser. Under these measuring conditions, the detection limit lies at <1 ppm (weight-ppm) carbon.

A quartz glass boat which is suitable for this analysis using the above named carbon analyser is obtainable as a consumable for the LECO analyser with LECO number 781-335 on the laboratory supplies market, in the present case from Deslis Laborhandel, Flurstrae 21, D-40235 Dusseldorf (Germany), Deslis-No. LQ-130XL. Such a boat has width/length/height dimensions of ca. 25 mm/60 mm/15 mm. The quartz glass boat is filled up to half its height with sample material. For silicon dioxide powder, a sample weight of 1.0 g sample material can be reached. The lower detection limit is then <1 weight ppm carbon. In the same boat, a sample weight of 4 g of a silicon dioxide granulate is reached for the same filling height (mean particle size in the range from 100 to 500 μm). The lower detection limit is then about 0.1 weight ppm carbon. The lower detection limit is reached when the measurement surface integral of the sample is not greater than three times the measurement surface integral of an empty sample (empty sample=the above process but with an empty quartz glass boat).

1. Curl Parameter

The curl parameter (also called: “Fibre Curl”) is measured according to DIN EN 60793-1-34:2007-01 (German version of the standard IEC 60793-1-34:2006). The measurement is made according to the method described in Annex A in the sections A.2.1, A.3.2 and A.4.1 (“extrema technique”).

m. Attenuation

The attenuation is measured according to DIN EN 60793-1-40:2001 (German version of the standard IEC 60793-1-40:2001). The measurement is made according to the method described in the annex (“cut-back method”) at a wavelength of λ=1550 nm.

n. Viscosity of the Slurry

The slurry is set to a concentration of 30 weight-% solids content with demineralised water (Direct-Q 3UV, Millipore, Water quality: 18.2 MΩcm). The viscosity is then measured with a MCR102 from Anton-Paar. For this, the viscosity is measured at 5 rpm. The measurement is made at a temperature of 23° C. and an air pressure of 1013 hPa.

o. Thixotropy

The concentration of the slurry is set to a concentration of 30 weight-% of solids with demineralised water (Direct-Q 3UV, Millipore, water quality: 18.2 MΩcm). The thixotropy is then measured with an MCR102 from Anton-Paar with a cone and plate arrangement. The viscosity is measured at 5 rpm and at 50 rpm. The quotient of the first and the second value gives the thixotropic index. The measurement is made at a temperature of 23° C.

p. Zeta Potential of the Slurry

For zeta potential measurements, a zeta potential cell (Flow Cell, Beckman Coulter) is employed. The sample is dissolved in demineralised water (Direct-Q 3UV, Millipore, water quality: 18.2 MΩcm) to obtain a 20 mL solution with a concentration of 1 g/L. The pH is set to 7 through addition of HNO₃ solutions with concentrations of 0.1 mol/L and 1 mol/L and an NaOH solution with a concentration of 0.1 mol/L. The measurement is made at a temperature of 23° C.

q. Isoelectric Point of the Slurry

The isoelectric point, a zeta potential measurement cell (Flow Cell, Beckman Coulter) and an auto titrator (DelsaNano AT, Beckman Coulter) is employed. The sample is dissolved in demineralised water (Direct-Q 3UV, Millipore, water quality: 18.2 MΩcm) to obtain a 20 mL solution with a concentration of 1 g/L. The pH is varied by adding HNO₃ solutions with concentrations of 0.1 mol/L and 1 mol/L and an NaOH solution with a concentration of 0.1 mol/L. The isoelectric point is the pH value at which the zeta potential is equal to 0. The measurement is made at a temperature of 23° C.

r. pH Value of the Slurry

The pH value of the slurry is measured using a WTW 3210 from Wissenschaftlich-Technische-Werkstätten GmbH. The pH 3210 Set 3 from WTW is employed as electrode. The measurement is made at a temperature of 23° C.

s. Solids Content

A weighed portion m₁ of a sample is heated for 4 hours to 500° C. reweighed after cooling (m₂). The solids content w is given as m₂/m₁*100 [Wt. %].

t. Bulk Density

The bug: density is measured according to the standard DIN ISO 697:1984-01 with an SMG 697 from Powtec. The bulk material (silicon dioxide powder or granulate) does not clump.

u. Tamped Density (Granulate)

The tamped density is measured according to the standard DIN ISO 787:1995-10.

v. Measurement of the Pore Size Distribution

The pore size distribution is measured according to DIN 66133 (with a surface tension of 480 mN/m and a contact angle of 140°). For the measurement of pore sizes smaller than 3.7 nm, the Pascal 400 from Porotec is used. For the measurement of pore sizes from 3.7 nm to 100 μm, the Pascal 140 from Porotec is used. The sample is subjected to a pressure treatment prior to the measurement. For this a manual hydraulic press is used (Order-Nr. 15011 from Specac Ltd., River House, 97 Cray Avenue, Orpington, Kent BR5 4HE, U.K.). 250 mg of sample material is weighed into a pellet die with 13 mm inner diameter from Specac Ltd. and loaded with 1 t, as per the display. This load is maintained for 5 s and readjusted if necessary. The load on the sample is then released and the sample is dried for 4 h at 105±2° C. in a recirculating air drying cabinet.

The sample is weighed into the penetrometer of type 10 with an accuracy of 0.001 g and in order to give a good reproducibility of the measurement it is selected such that the stem volume used, i.e. the percentage of potentially used Hg volume for filling the penetrometer is in the range between 20% to 40% of the total Hg volume. The penetrometer is then slowly evacuated to 50 μm Hg and left at this pressure for 5 min. The following parameters are provided directly by the software of the measuring device: total pore volume, total pore surface area (assuming cylindrical pores), average pore radius, modal pore radius (most frequently occurring pore radius), peak n. 2 pore radius (μm).

w. Primary Particle Size

The primary particle size is measured using a scanning electron microscope (SEM) model Zeiss Ultra 55. The sample is suspended in demineralised water (Direct-Q 3UV, Millipore, water quality: 18.2 MΩcm), to obtain an extremely dilute suspension. The suspension is treated for 1 min with the ultrasound probe (UW 2070, Bandelin electronic, 70 W, 20 kHz) and then applied to a carbon adhesive pad.

x. Mean Particle Size in Suspension

The mean particle size in suspension is measured using a Mastersizer 2000, available from Malvern Instruments Ltd., UK, according to the user manual, using the laser deflection method. The sample is suspended in demineralised water (Direct-Q 3UV, Millipore, water quality: 18.2 MΩcm) to obtain a 20 mL suspension with a concentration of 1 g/L. The suspension is treated with the ultrasound probe (UW 2070, Bandelin electronic, 70 W, 20 kHz) for 1 min.

y. Particle Size and Core Size of the Solid

The particle size and core size of the solid are measured using a Camsizer XT, available from Retsch Technology GmbH, Deutschland according to the user manual The software gives the D10, D50 and D90 values for a sample.

z. BET Measurement

For the measurement of the specific surface area, the static volumetric BET method according to DIN ISO 9277:2010 is used. For the BET measurement, a “NOVA 3000” or a “Quadrasorb” (available from Quantachrome), which operate according to the SMART method (“Sorption Method with Adaptive dosing Rate”), is used. The micropore analysis is performed using the t-plot process (p/p0=0.1-0.3) and the mesopore analysis is performed using the MBET process (p/p0=0.0-0.3). As reference material, the standards alumina. SARM-13 and SARM-214, available from Quantachrome are used. The tare weight of the measuring cell (clean and dry) is weighed. The type of measuring cell is selected such that the sample material which is introduced and the filler rod fill the measuring cell as much as possible and the dead space is reduced to a minimum. The sample material is introduced into the measuring cell. The amount of sample material is selected so that the expected value of the measurement value corresponds to 10-20 m²/g. The measuring cells are fixed in the baking positions of the BET measuring device (without filler rod) and evacuated to <200 mbar. The speed of the evacuation is set so that no material leaks from the measuring cell. Baking is performed in this state at 200° C. for lh. After cooling, the measuring cell filled with the sample is weighed (raw value). The tare weight is then subtracted from the raw value of the weight=nett weight=weight of the sample. The filling rod is then introduced into the measuring cell this is again fixed at the measuring location of the BET measuring device. Prior to the start of the measurement, the sample identifications and the sample weights are entered into the software. The measurement is started. The saturation pressure of nitrogen gas (N2 4.0) is measured. The measuring cell is evacuated and cooled down to 77 K using a nitrogen bath. The dead space is measured using helium gas (He 4.6). The measuring cell is evacuated again. A multi point analysis with at least 5 measuring points is performed. N2 4.0 is used as absorptive. The specific surface area is given in m²/g.

za. Viscosity of Glass Bodies

The viscosity of the glass is measured using the beam bending viscosimeter of type 401—from TA Instruments with the manufacturer's software WinTA (current version 9.0) in Windows 10 according to the DIN ISO 7884-4:1998-02 standard. The support width between the supports is 45 mm. Sample rods with rectangular cross section are cut from regions of homogeneous material (top and bottom sides of the sample have a finish of at least 1000 grain). The sample surfaces after processing have a grain size=9 μm & RA=0.15 μm. The sample rods have the following dimensions: length=50 mm, width=5 mm & height=3 mm (ordered: length, width, height, as in the standards document). Three samples are measured and the mean is calculated. The sample temperature is measured using a thermocouple tight against the sample surface. The following parameters are used: heating rate=25 K up to a maximum of 1500° C., loading weight=100 g, maximum bending=3000 μm (deviation from the standards document).

zb. Dew Point Measurement

The dew point is measured using a dew point mirror hygrometer called “Optidew” of the company Michell Instruments GmbH, D-61381 Friedrichsdorf. The measuring cell of the dew point mirror hygrometer is arranged at a distance of 100 cm from the gas outlet of the oven. For this, the measuring device with the measuring cell is connected in gas communication to the gas outlet of the oven via a T-piece and a hose (Swagelok PFA, Outer diameter: 6mm). The over pressure at the measuring cell is 10±2 mbar. The through flow of the gas through the measuring cell is 1-2 standard litre/min. The measuring cell is in a room with a temperature of 25° C., 30% relative air humidity and a mean pressure of 1013 hPa.

zc. Residual Moisture (Water Content)

The measurement of the residual moisture of a sample of silicon dioxide granulate is performed using a Moisture Analyzer HX204 from Mettler Toledo. The device functions using the principle of thermogravimetly. The HX204 is equipped with a halogen light source as heating element. The drying temperature is 220° C. The starting weight of the sample is 10 g±10%. The “Standard” measuring method is selected. The drying is carried out until the weight change reaches not more than 1 mg/140 s. The residual moisture is given as the difference between the initial weight of the sample and the final weight of the sample, divided by the initial weight of the sample.

The measurement of residual moisture of silicon dioxide powder is performed according to DIN EN ISO 787-2:1995 (2 h, 105° C.).

EXAMPLES

The example is further illustrated in the following through examples. The invention is not limited by the examples.

A. 1. Preparation of Silicon Dioxide Powder (OMCTS Route)

An aerosol formed by atomising a siloxane with air (A) is introduced under pressure into a flame which is formed by igniting a mixture of oxygen enriched air (B) and hydrogen. Furthermore, a gas flow (C) surrounding the flame is introduced and the process mixture is then cooled with process gas. The product is separated off at a filter. The process parameters are given in Table 1 and the specifications of the resulting product are given in Table 2. Experimental data for this example are indicated with A1-x.

2. Modification 1: Increased Carbon Content

A process was carried out as described in A.1., but the burning of the siloxane was performed in such a way that an amount of carbon was also formed. Experimental data for this example are indicated with A2-x.

TABLE 1 Example A1-1 A2-1 A2-2 Aerosol formation Siloxane OMCTS* OMCTS* OMCTS* Feed rate kg/h 10 10 10 (kmol/h) (0.0337) (0.0337) (0.0337) Feed rate of air (A) Nm³/h 14 10 12 Pressure barO 1.2 1.2 1.2 Burner feed Oxygen enriched air (B) Nm³/h 69 65 68 O₂-content Vol. % 32 30 32 total O₂ feed rate Nm³/h 25.3 21.6 24.3 kmol/h 1.130 0.964 1.083 Hydrogen feed rate Nm³/h 27 27 12 kmol/h 1.205 1.205 0.536 Feed — — Carbon compound Material methane Amount Nm³/h 5.5 Air flow (C) Nm³/h 60 60 60 Stoichiometric ratio V 2.099 1.789 2.011 X 0.938 0.80 2.023 Y 0.991 0.845 0.835 V = molar ratio of employed O₂/O₂ required for completed oxidation of the siloxane; X = molar ratio O₂/H₂; Y = (molar ratio of employed O₂/O₂ required for stoichiometric conversion OMCTS + fuel gas); barO = overpressure; *OMCTS = Octamethylcyclotetrasiloxane.

TABLE 2 Example A1-1 A2-1 A2-2 BET m²/g 30 33 34 Bulk density g/ml 0.114 +− 0.011 0.105 +− 0.011 0.103 +− 0.011 tamped density g/ml 0.192 +− 0.015 0.178 +− 0.015 0.175 +− 0.015 Primary particle size nm 94 82 78 particle size distribution D10 μm 3.978 ± 0.380 5.137 ± 0.520 4.973 ± 0.455 particle size distribution D50 μm 9.383 ± 0.686 9.561 ± 0.690 9.423 ± 0.662 particle size distribution D90 μm 25.622 ± 1.387  17.362 ± 0.921  18.722 ± 1.218  C content ppm 34 ± 4  73 ± 6  80 ± 6  Cl content ppm <60 <60 <60 Al content ppb 20 20 20 Total content of metals other than Al ppb <700 <700 <700 residual moisture content wt.-% 0.02-1.0 0.02-1.0 0.02-1.0 pH value in water 4% (IEP) — 4.8 4.6 4.5 Viscosity at 5 rpm, aqueous mPas 753 1262 1380 suspension 30 Wt-%, 23° C. Alkali earth metal content ppb 538 487 472

B. 1. Preparation of Silicon Dioxide Powder (Silicon Source: SiCl₄)

A portion of silicon tetrachloride (SiCl₄) is evaporated at a temperature T and introduced with a pressure P into a flame of a burner which is formed by igniting a mixture of oxygen enriched air and hydrogen. The mean normalised gas flow to the outlet is held constant. The process mixture is then cooled with process gas. The product was separated off at a filter. The process parameters are given in Table 3 and the specifications of the resulting products are given in Table 4. They are indicated with B1-x.

2. Modification 1: Increased Carbon content

A process was carried out as described in B.1., but the burning of the silicon tetrachloride was performed such that an amount of carbon was also formed. Experimental data for this example are indicated with B2-x.

TABLE 3 Example B1-1 B2-1 Aerosol formation Silicon tetrachloride feed kg/h 50 50 (kmol) (0.294) (0.294) Temperature T ° C. 90 90 Pressure p barO 1.2 1.2 Burner feed Oxygen enriched air, Nm³/h 145 115 O₂ content therein Vol. % 45 30 Feed — Carbon compound Material methane Amount Nm³/h 2.0 Hydrogen feed Nm³/h 115 60 kmol/h 5.13 2.678 Stoichiometric ratios X 0.567 0.575 Y 0.946 0.85 X = as molar ratio O₂/H₂; Y = molar ratio of employed O₂/O₂ required for stoichiometric reaction with SiCl4 + H2 + CH4); barO = Overpressure.

TABLE 4 Example B1-1 B2-1 BET m²/g 49 47 Bulk density g/ml 0.07 ± 0.01 0.06 ± 0.01 tamped density g/ml 0.11 ± 0.01 0.10 ± 0.01 Primary particle size nm 48 43 particle size distribution D10 μm 5.0 ± 0.5 4.5 ± 0.5 particle size distribution D50 μm 9.3 ± 0.6 8.7 ± 0.6 particle size distribution D90 μm 16.4 ± 0.5  15.8 ± 0.7  C content ppm <4 76 Cl content ppm 280 330 Al content ppb 20 20 Total of the concentrations of ppb <1300 <1300 Ca, Co, Cr, Cu, Fe, Ge, Hf, K, Li, Mg, Mn, Mo, Na, Nb, Ni, Ti, V, W, Zn, Zr residual moisture content wt.-% 0.02-1.0 0.02-1.0 pH value in water 4% (IEP) pH 3.8 3.8 Viscosity at 5 rpm, aqueous mPas 5653 6012 suspension 30 Wt-%, 23° C. Alkali earth metal content ppb 550 342

C. Steam Treatment

A particle flow of silicon dioxide powder is introduced via the top of a standing column. Steam at a temperature (A) and air are fed via the bottom of the column. The column is maintained at a temperature (B) at the top of the column and at a second temperature (C) at the bottom of the column by an internally situated heater. After leaving the column (holding time (D)) the silicon dioxide powder has in particular the properties shown in Table 6. The process parameters are given in Table 5.

TABLE 5 Example C-1 C-2 Educt: Product of B1-1 B2-1 Educt feed kg/h 100 100 Steam feed kg/h 5 5 Steam temperature (A) ° C. 120 120 Air feed Nm³/h 4.5 4.5 Column height m 2 2 Inner diameter mm 600 600 T (B) ° C. 260 260 T (C) ° C. 425 425 Holding time (D) silicon s 10 10 dioxide powder

TABLE 6 Example C-1 C-2 pH value in water 4% (IEP) — 4.6 4.6 Cl content ppm <60 <60 C content ppm <4 36 Viscosity at 5 rpm, aqueous mPas 1523 1478 suspension 30 Wt-%, 23° C.

The silicon dioxide powders obtained in the examples C-1 and C-2 each have a low chlorine content as well as a moderate pH value in suspension. The carbon content of example C-2 is higher as for C-1.

D. Treatment with a Neutralising Agent

A particle flow of silicon dioxide powder is introduced via the top of a standing column. A neutralising agent and air are fed via the bottom of the column. The column is maintained at a temperature (B) at the top of the column and at a second temperature (C) at the bottom of the column by an internally situated heater. After leaving the column (holding time (D)) the silicon dioxide powder has in particular the properties shown in Table 8. The process parameters are given in Table 7.

TABLE 7 Example D-1 Educt: Product from B1-1 Educt feed kg/h 100 Neutralising agent Ammonia Neutralising agent feed kg/h 1.5 Neutralising agent Obtainable from Air Liquide: specifications Ammonia N38, purity ≥ 99.98 Vol. % Air feed Nm³/h 4.5 Column height m 2 inner diameter mm 600 T (B) ° C. 200 T (C) ° C. 250 Holding time (D) of s 10 silicon dioxide powder

TABLE 8 Example D-1 pH value in water 4% (IEP) — 4.8 Cl content ppm 210 C content ppm <4 Viscosity at 5 rpm, aqueous mPas 821 suspension 30 Wt-%, 23° C.

E. 1. Preparation of Silicon Dioxide Granulate from Silicon Dioxide Powder

A silicon dioxide powder is dispersed in fully desalinated water. For this, an intensive mixer of type R from the Gustav Eirich machine factory is used. The resulting suspension is pumped with a membrane pump and thereby pressurised and converted into droplets by a nozzle. These are dried in a spray tower and collect on the floor of the tower. The process parameters are given in Table 9 and the properties of the obtained granulate in Table 10. Experimental data for this example are indicated with E1-x.

2. Modification 1: Increased Carbon Content

The process is analogous to that described in E.1. Additionally, carbon powder is dispersed into the suspension. Experimental data for these examples are indicated with E2-x.

3. Modification 2: Addition of Silicon

The process is analogous to that described in E.1. Additionally, a silicon component is dispersed into the suspension. Experimental data for these examples are identified with E3-1.

TABLE 9 Example E1-1 E1-2 E1-3 E1-4 E1-5 E2-1 E3-1 E3-2 Educt = Product from A1-1 A2-1 B1-1 C-1 C-2 A1-1 A1-1 A2-1 Amount of educt Kg 10 10 10 10 10 10 10 10 Carbon powder Material — — — — — C** — — Max. Particle size  75 μm Amount 1 g Silicon component Material — — — — — — silicon — powder*** Grain size (d50) 8 μm Amount 1000 ppm Carbon content 0.5 ppm Total of the concentra- 5 ppm tions of Ca, Co, Cr, Cu, Fe, Ge, Hf, K, Li, Mg, Mn, Mo, Na, Nb, Ni, Ti, V, W, Zn, Zr Water Rating* FD FD FD FD FD FD FD FD Litre 5.4 5.4 5.4 5.4 5.4 5.4 5.4 5.4 Dispersion Solids content Wt. % 65 65 65 65 65 65 65 65 Nozzle Diameter mm 2.2 2.2 2.2 2.2 2.2 2.2 2.2 2.2 Temperature ° C. 25 25 25 25 25 25 25 25 Pressure Bar 16 16 16 16 16 16 16 16 Installation height m 6.5 6.5 6.5 6.5 6.5 6.5 6.5 6.5 Spray tower Height m 18.20 18.20 18.20 18.20 18.20 18.20 18.20 18.20 Inner diameter m 6.30 6.30 6.30 6.30 6.30 6.30 6.30 6.30 T (introduced gas) ° C. 380 380 380 380 380 380 380 380 T (exhaust) ° C. 110 110 110 110 110 110 110 110 Air flow m³/h 6500 6500 6500 6500 6500 6500 6500 6500 Installation height = distance between nozzle and lowest point of the spray tower interior in the direction of gravity. *FD = fully desalinated, conductance ≤ 0.1 μS; **C 006011: Graphite powder, max. particle size: 75 μm, high purity (available from Goodfellow GmbH, Bad Nauheim (Germany); ***available from Wacker Chemie AG (Munich, Germany).

TABLE 10 Example E1-1 E1-2 E1-3 E1-4 E1-5 E2-1 E3-1 E3-2 BET m²/g 30 33 49 49 47 28 31 35 Bulk density g/ml 0.8 ± 0.1 0.8 ± 0.1 0.8 ± 0.1 0.8 ± 0.1 0.8 ± 0.1 0.8 ± 0.1 0.8 ± 0.1 0.8 ± 0.1 tamped density g/ml 0.9 ± 0.1 0.9 ± 0.1 0.9 ± 0.1 0.9 ± 0.1 0.9 ± 0.1 0.9 ± 0.1 0.9 ± 0.1 0.9 ± 0.1 mean particle size μm 255 255 255 255 255 255 255 255 particle size distribu- μm 110 110 110 110 110 110 110 110 tion D10 particle size distribu- μm 222 222 222 222 222 222 222 222 tion D50 particle size distribu- μm 390 390 390 390 390 390 390 390 tion D90 SPHT3 Dim- 0.64-0.98 0.64-0.98 0.64-0.98 0.64-0.98 0.64-0.98 0.64-0.98 0.64-0.98 0.64-0.98 less Aspect ratio W/L Dim- 0.64-0.94 0.64-0.94 0.64-0.94 0.64-0.94 0.64-0.94 0.64-0.94 0.64-0.94 0.64-0.94 (width to length) less C content ppm <4 39 <4 <4 32 100 <4 39 Cl content ppm <60 <60 280 <60 <60 <60 <60 <60 Al content ppb 20 20 20 20 20 20 20 20 Total of the concentra- ppb <700 <700 <1300 <1300 <1300 <700 <700 <700 tions of Ca, Co, Cr, Cu, Fe, Ge, Hf, K, Li, Mg, Mn, Mo, Na, Nb, Ni, Ti, V, W, Zn, Zr residual moisture con- wt.-% <3 <3 <3 <3 <3 <3 <3 <3 tent Alkaline earth metal ppb 538 487 550 550 342 538 538 487 content pore volume ml/g 0.33 0.33 0.45 0.45 0.45 0.33 0.33 0.33 angle of repose ° 26 26 26 26 26 26 26 26

The granulates are all open pored, have a uniform and spherical shape (all by microscopic investigation). They tend not to stick together or cement.

F. Cleaning of Silicon Dioxide Granulate

Silicon dioxide granulate is first optionally treated with oxygen or nitrogen (see Table 11) at a temperature T1. Subsequently, the silicon dioxide granulate is treated with a co-flow of a chlorine containing component, wherein the temperature is raised to a temperature T2. The process parameters are given in Table 11 and the properties of the obtained treated granulate in Table 12.

TABLE 11 Example F1-1 F1-2 F1-3 F1-4 F1-5 F2-1 F3-1 F3-2 Educt = Product from E1-1 E1-2 E1-3 E1-4 E1-5 E2-1 E3-1 E3-2 Rotary kiln ¹⁾ length cm 200 200 200 200 200 200 Inner diameter cm 10 10 10 10 10 10 Throughput kg/h 2 2 2 2 2 2 Rotational speed rpm 2 2 2 2 2 2 T1 ° C. 1100 NA 1100 1100 1100 NA 1100 1100 Atmosphere O2 pure NA O2 pure O2 pure O2 pure NA N2 N2 Reactant O2 NA O2 O2 O2 NA None None Feed 300 l/h NA 300 l/h 300 l/h 300 l/h NA residual moisture wt.-% <1 <3 <1 <1 <1 <3 <1 <1 content T2 ° C. 1100 1100 1100 1100 1100 1100 NA NA Co-flow Component 1: HCl l/h 50 50 50 50 50 50 NA NA Component 2: Cl2 l/h 0 15 0 0 0 15 NA NA Component 3: N2 l/h 50 35 50 50 50 35 NA NA Total co-flow l/h 100 100 100 100 100 100 NA NA ¹⁾ For the rotary kilns, the throughput is selected as the control variable. That means that during operation the mass flow exiting from the rotary kiln is weighed and then the rotational speed and/or the inclination of the rotary kiln is adapted accordingly. For example, an increase in the throughput can be achieved by a) increasing the rotational speed, or b) increasing the inclination of the rotary kiln away from horizontal, or a combination of a) and b).

TABLE 12 Example F1-1 F1-2 F1-3 F1-4 F1-5 F2-1 F3-1 F3-2 BET m²/g 25 27 43 45 40 23 25 26 C content ppm <4 <4 <4 <4 <4 <4 <4 <4 Cl content ppm 100-200 100-200 300-400 100-200 100-200 100-200 <60 <60 Al content ppb 20 20 20 20 20 20 20 20 pore volume mm³/g 650 650 650 650 650 650 650 650 Total of the ppb <200 <200 <200 <200 <200 <200 <700 <700 concentrations of Ca, Co, Cr, Cu, Fe, Ge, Hf, K, Li, Mg, Mn, Mo, Na, Nb, Ni, Ti, V, W, Zn, Zr Alkaline earth ppb 115 55 95 115 40 35 136 33 metal content tamped density g/cm³ 0.95 ± 0.05 0.95 ± 0.05 0.95 ± 0.05 0.95 ± 0.05 0.95 ± 0.05 0.95 ± 0.05 0.95 ± 0.05 0.95 ± 0.05 In the case of F1-2, F2-1 and F3-2, the granulates after the cleaning step show a significantly reduced carbon content (like low carbon granulate, e.g. F1-1). In particular, F1-2, F1-5, F2-1 and F3-2 show a significantly reduced content of alkaline earth metals. SiC formation was not observed.

G. Treatment of Silicon Dioxide Granulate by Warming

Silicon dioxide granulate is subjected to a temperature treatment in a pre chamber in the form of a rotary kiln which is positioned upstream of the melting oven and which is connected in flow connection to the melting oven via a further intermediate chamber. The rotary kiln is characterised by a temperature profile which increases in the flow direction. A further treated silicon dioxide granulate was obtained. In example G-4-2 no treatment by warming was performed during mixing in the rotary kiln. The process parameters are given in Table 13 and the properties of the obtained treated granulate in Table 14.

TABLE 13 Example G1-1 G1-2 G1-3 G1-4 G1-5 G2-1 G3-1 G3-2 G4-1 G4-2 Educt = Product from F1-1 F1-2 F1-3 F1-4 F1-5 F2-1 F3-1 F3-2 F1-1 F1-1 Silicon components Material — — — — — — — — Silicon Silicon powder*** powder*** Amount 0.01% 0.1% Rotary kiln ¹⁾ Length cm 200 200 200 200 200 200 200 200 200 NA Inner diameter cm  10 10 10 10 10 10 10 10 10 Throughput kg/h  8 5 5 5 5 5 5 5 5 Rotation speed rpm  30 30 30 30 30 30 30 30 30 T1 (Rotary kiln inlet) ° C. RT RT RT RT RT RT RT RT RT T2 (Rotary kiln outlet) ° C. 500 500 500 500 500 500 500 500 500 Atmosphere Gas, flow air, free O₂, in O₂, in O₂, in O₂, in O₂, in O₂, in O₂, in O₂, in direction convec- contra- contra- contra- contra- contra- contra- contra- contra- tion flow flow flow flow flow flow flow flow Total throughput Nm³/h 0.6 0.6 0.6 0.6 0.6 0.6 0.6 0.6 of gas flow ***Grain size D₅₀ = 8 μm; carbon content ≤ 5 ppm; Total foreign metals ≤ 5 ppm; 0.5 ppm; available from Wacker Chemie AG (Munich, Germany). RT = room temperature (25° C.) ¹⁾ For the rotary kilns, the throughput is selected as the control variable. That means that during operation the mass flow exiting from the rotary kiln is weighed and then the rotational speed and/or the inclination of the rotary kiln is adapted accordingly. For example, an increase in the throughput can be achieved by a) increasing the rotational speed, or b) increasing the inclination of the rotary kiln away from horizontal, or a combination of a) and b).

TABLE 14 Example G1-1 G1-2 G1-3 G1-4 G1-5 G2-1 G3-1 G3-2 G4-1 G4-2 BET m²/g 22 23 38 42 37 22 22 21 22 24 Water content (residual moisture) ppm 500 100 100 100 100 100 500 100 500 <10000 C content ppm <4 <4 <4 <4 <4 <4 <4 <4 <4 <4 Cl content ppm 100-200 100-200 300-400 100-200 100-200 100-200 <60 <60 100-200 100-200 Al content ppb 20 20 20 20 20 20 20 20 20 20 Total of the concentrations of Ca, Co, Cr, ppb ≤200 ≤200 ≤200 ≤200 ≤200 ≤200 ≤200 ≤200 ≤200 ≤200 Cu, Fe, Ge, Hf, K, Li, Mg, Mn, Mo, Na, Nb, Ni, Ti, V, W, Zn, Zr Alkaline earth metal content ppb 115 55 95 115 40 35 136 33 115 115 angle of repose ° 26 26 26 26 26 26 26 26 26 26 Due to this treatment, G3-1 and G3-2 exhibit a significantly reduced alkaline earth metal content in comparison to before (E3-1 & E3-2 respectively).

H. Melting of Granulate to Obtain Quartz Glass

Silicon dioxide granulate according to line 2 of Table 15 is employed for preparing a quartz glass tube in a vertical crucible drawing process. The structure of the standing oven, for example H5-1 comprising a standing melting crucible is shown schematically in FIG. 7, and for all the other examples with a hanging melting crucible FIG. 6 serves as a schematic representation. The silicon dioxide granulate is introduced via the solids feed and the interior of the melting crucible is flushed with a gas mixture. In the melting crucible, a glass melt forms upon which a reposing cone of silicon dioxide granulate sits. In the lower region of the melting crucible, molten glass is removed from the glass melt through a die (optionally with a mandrel) and is pulled vertically down in the form of a tubular thread. The output of the plant results from the weight of the glass melt, the viscosity of the glass through the nozzle the size of the hole provided by the nozzle. By varying the feed rate of silicon dioxide granulate and the temperature, the output can be set to the desired level. The process parameters are given in Table 15 and Table 17 and in some cases in Table 19 and the properties of the formed quartz glass body in Table 16 and Table 18.

In Example H7-1, a gas distributing ring is arranged in the melting crucible, with which the flushing gas is fed close to the surface of the glass melt. An example of such an arrangement is shown in FIG. 8.

In Example H8-x,, the dew point is measured at the gas outlet. The measuring principle is shown in FIG. 10. Between the outlet of the melting crucible and the measuring location of the dew point, the gas flow covers a distance of 100 cm.

TABLE 15 Example H1-1 H1-2 H1-3 H1-4 H1-5 H3-1 H3-2 H4-1 H4-2 Educt = Product from G1-1 G1-2 G1-3 G1-4 G1-5 G3-1 G3-2 G4-1 G4-2 Melting crucible Type Hanging Hanging Hanging Hanging Hanging Hanging Hanging Hanging Hanging metal metal metal metal metal metal metal metal metal sheet sheet sheet sheet sheet sheet sheet sheet sheet crucible crucible crucible crucible crucible crucible crucible crucible crucible Type of metal tungsten tungsten tungsten tungsten tungsten tungsten tungsten tungsten tungsten length cm 200 150 150 150 150 200 150 200 200 Inner diameter cm 40 25 25 25 25 40 25 40 40 Throughput kg/h 30 20 20 20 20 30 20 30 30 T1 (Gas compartment of the ° C. 300 300 300 300 300 300 300 300 300 melting crucible) T2 (glass melt) ° C. 2100 2100 2100 2100 2100 2100 2100 2100 2100 T3 (nozzle) ° C. 1900 1900 1900 1900 1900 1900 1900 1900 1900 Atmosphere/Flushing gas He Vol.-% 50 50 50 50 50 50 50 50 50 Concentration H₂ Vol.-% 50 50 50 50 50 50 50 50 50 Concentration Total gas flow throughput Nm³/h 4 4 4 4 4 2 4 2 2 O₂ ppm ≤100 ≤100 ≤100 ≤100 ≤100 ≤100 ≤100 ≤100 ≤100

TABLE 16 Example H1-1 H1-2 H1-3 H1-4 H1-5 H3-1 H3-2 H4-1 H4-2 C content ppm <4 <4 <4 <4 <4 <4 <4 <4 <4 Cl content ppm 100-200 100-200 300-400 100-200 100-200 <60 <60 100-200 100-200 Al content ppb 20 20 20 20 20 20 20 20 20 Total of the ppb <400 <400 <400 <400 <400 <400 <400 <400 <400 concentrations of Ca, Co, Cr, Cu, Fe, Ge, Hf, K, Li, Mg, Mn, Mo, Na, Nb, Ni, Ti, V, W, Zn, Zr OH content ppm 400 400 400 400 400 80 400 80 80 Alkaline ppb 115 55 95 115 40 136 33 115 115 earth metal content ODC content l/cm³ 4*10¹⁵ 2*10¹⁶ 4*10¹⁵ 4*10¹⁵ 4*10¹⁵ 5*10¹⁸ 2*10¹⁶ 5*10¹⁸ 8*10¹⁸ pore volume mL/g 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1 Outer diameter cm 19.7 3.0 19.7 19.7 19.7 19.7 3.0 19.7 19.7 tubular thread/quartz glass body Viscosity @1250° C. Lg(η/dPas) 11.69 ± 11.69 ± 11.69 ± 11.69 ± 11.69 ± 12.16 ± 11.69 ± 12.16 ± 12.16 ± 0.13 0.13 0.13 0.13 0.13 0.2 0.13 0.2 0.2 @1300° C. 11.26 ± 11.26 ± 11.26 ± 11.26 ± 11.26 ± 11.49 ± 11.26 ± 11.49 ± 11.49 ± 0.1 0.1 0.1 0.1 0.1 0.15 0.1 0.15 0.15 @1350° C. 10.69 ± 10.69 ± 10.69 ± 10.69 ± 10.69 ± 10.88 ± 10.69 ± 10.88 ± 10.88 ± 0.07 0.07 0.07 0.07 0.07 0.1 0.07 0.1 0.1 “±”-value are the standard deviation.

TABLE 17 Example H5-1 H6-1 H7-1 H8-1 H8-2 H8-3 H8-4 Educt = Product from G1-1 G1-1 G1-1 G1-1 G1-1 G1-1 G1-1 Melting crucible Type Standing Hanging Hanging Hanging Hanging Hanging Hanging sinter sinter metal plate metal plate metal plate metal plate metal plate crucible crucible crucible crucible crucible crucible crucible Type of metal tungsten tungsten tungsten tungsten tungsten tungsten tungsten Additional fittings — — Gas dis- dew point dew point dew point dew point and fixtures tributor measure- measure- measure- measure- ring ment ment ment ment Length cm 250 250 200 200 200 200 200 Inner diameter cm 40 36 40 40 40 40 40 Throughput kg/h 40 35 30 30 30 30 30 T1 (Gas compartment of ° C. 300 400 300 300 300 300 300 melting crucible) T2 (glass melt) ° C. 2100 2150 2100 2100 2100 2100 2100 T3 (Nozzle) ° C. 1900 1900 1900 1900 1900 1900 1900 Atmosphere He Vol.-% 30 50 50 50 50 50 50 Concentration H₂ Vol.-% 70 50 50 50 50 50 50 Concentration Total gas flow Nm³/h 4 4 8 8 4 3 2 throughput O₂ ppm <100 <100 ≤10 ≤100 ≤100 ≤100 ≤100

TABLE 18 Example H5-1 H6-1 H7-1 H8-1 H8-2 H8-3 H8-4 C content ppm <4 <4 <4 <4 <4 <4 <4 Cl content ppm 100-200 100-200 100-200 100-200 100-200 100-200 100-200 Al content ppb 20 20 20 20 20 20 20 Total of the concentrations of ppb <400 <400 <400 <400 <400 <400 <400 Ca, Co, Cr, Cu, Fe, Ge, Hf, K, Li, Mg, Mn, Mo, Na, Nb, Ni, Ti, V, W, Zn, Zr OH content ppm 400 400 400 250 400 500 800 Alkaline earth metal content ppb 115 115 115 115 115 115 115 ODC content l/cm³ <4*10¹⁵ <4*10¹⁵ <4*10¹⁵ <4*10¹⁵ <4*10¹⁵ <4*10¹⁵ <4*10¹⁵ Content of W, Mo, Re, Ir, Os ppb <300 ppb <300 ppb <100 ppb <50 ppb <100 ppb <5 ppm 100 ppm Outer diameter of tubular cm 26.0 19.7 19.7 19.7 19.7 19.7 19.7 thread/quartz glass body Viscosity @1250° C. lg(η/dPas) 11.69 ± 0.13 11.69 ± 0.13 11.69 ± 0.13 12.06 ± 0.15 11.69 ± 0.13 11.69 ± 0.13 11.63 ± 0.13 @1300° C. 11.26 ± 0.1  11.26 ± 0.1  11.26 ± 0.1  11.38 ± 0.1  11.26 ± 0.1  11.26 ± 0.1  11.22 ± 0.1  @1350° C. 10.69 ± 0.07 10.69 ± 0.07 10.69 ± 0.07 10.75 ± 0.08 10.69 ± 0.07 10.69 ± 0.07 10.65 ± 0.07

TABLE 19 Example H-7-1 H8-1 H8-2 H8-3 H8-4 Distributor ring (Gas inlet in the melting cm 2 — — — — crucible), Height above the glass melt Location of gas outlet In the lid In the lid In the lid In the lid In the lid of the of the of the of the of the melting melting melting melting melting crucible crucible crucible crucible crucible Dew point of the gas flow Before introduction into melting −90 −90 −90 −90 −90 crucible After removal from melting crucible −10 −30 −10 0 +10

I. Post Processing of a Quartz Glass Body

A quartz glass body obtained in example H1-1 and which has already been drawn (1000 kg, Surface area=110 m²; Diameter=1.65 cm, Total length 2120 m) is cut into pieces with a length of 200 cm by scoring and striking. The end surfaces were post worked by sawing to obtain a flat end surface. The obtained batch of quartz glass bodies (N-1) was cleaned by dipping in an HF bath (V=2 m³) for 30 minutes and then rinsed with fully desalinated water (to obtain quartz glass body (N-1′)).

J. “Used Acid” (HF Bath After Use)

The liquid in the dipping bath in example I (V=2 m³) is tested directly after the treatment of the quartz glass body (I-1′) and without further treatment. The liquid employed for the above described treatment is characterised before and after the treatment by the properties given in Table 20.

TABLE 20 Before After treatment of a treatment quartz glass body of of a quartz mass m = 1000 kg Element Unit glass body and surface area of 110 m² Al ppm 0.04 0.8 Refractory metal ppm 0 0.15 (W, Mo, . . .) Further metals ppm 0.15 1 according to entire list * in total, of which Ca ppm 0.01 0.3 Mg ppm 0.04 0.09 Na ppm 0.04 0.1 Cr ppm 0.01 0.01 Ni ppm 0.001 0.01 Fe ppm 0.01 0.05 Zr ppm 0.01 0.05 Ti ppm 0.01 0.05 HF wt.-% 40 35 Content of Si—F wt.-% 4 6 compounds Density g/cm³ 1.14 1.123

K. Preparation of a Quartz Glass Grain

Quartz glass bodies with the features indicated in Table 21 are reduced in size to a quartz glass grain. For this, 100 kg of the quartz glass body is subjected to a so-called electrodynamic size reduction process. The starting glass is reduced in size in a tank by electrical pulses to the desired grain size. If necessary, the material is screened using a vibrating screen in order to separate off disrupting fine and course proportions. The quartz glass grain is flushed, acidified with HF, flushed again with water and dried. The quartz glass purified in this way has the properties indicated in Table 22.

TABLE 21 Example H1-1 H4-1 H3-1 H1-3 C content ppm <4 <4 <4 <4 Cl content ppm 100-200 100-200 <60 300-400 Al content ppb 20 20 20 20 Total of the concentrations of ppb <400 <400 <400 <400 Ca, Co, Cr, Cu, Fe, Ge, Hf, K, Li, Mg, Mn, Mo, Na, Nb, Ni, Ti, V, W, Zn, Zr OH content ppm 400 80 80 400 Viscosity @1250° C. Lg(η/dPas) 11.69 ± 0.13 12.16 ± 0.2 12.16 ± 0.2 11.69 ± 0.13 @1300° C. 11.26 ± 0.1   11.49 ± 0.15  11.49 ± 0.15 11.26 ± 0.1  @1350° C. 10.69 ± 0.07 10.88 ± 0.1 10.88 ± 0.1 10.69 ± 0.07

TABLE 22 Example I1-1 I4-1 I3-1 I1-3 Educt = Product from H1-1 H4-1 H3-1 H1-3 C content ppm <4 <4 <4 <4 Cl content ppm 100-200 100-200 100-200 300-400 Al content ppb 20 20 20 20 Total of the concentrations of ppb <1000 <1000 <1000 <1000 Ca, Co, Cr, Cu, Fe, Ge, Hf, K, Li, Mg, Mn, Mo, Na, Nb, Ni, Ti, V, W, Zn, Zr OH content ppb 400 80 80 400 BET cm²/g <1 <1 <1 <1 Bulk density g/cm³ 1.25 1.35 1.35 1.30 Particle size D10 mm 0.85 0.09 0.08 0.1 D50 mm 2.21 0.18 0.19 0.18 D90 mm 3.20 0.27 0.26 0.25

L. Preparation of an Opaque Body I

The previously prepared quartz glass grain I1-1 and I3-1 were each processed to form a slurry. For a batch of 10 kg of slurry (also: silicon dioxide water slurry), ca. 8.2 kg of the quartz glass grain is mixed with 1.8 kg deionised water with a conductivity of less than 3 μS in a tumbling mill lined with quartz glass, which mill has a volume of ca. 20 litres.

This mixture is further ground by grinding balls made of quartz glass on a roller block at 23 rpm for a period of 3 days until a homogeneous slurry forms which has a solids content of 79%. In the course of grinding, the pH falls to approximately 4.

The slurry is then left to run, bubble-free, into a mould which has been provided with air-extraction ducts, previously purified with compressed air, free from dust and oil, and demineralised water (conductivity <0.05 μS) with the dimensions of the recess: outer diameter=315 mm; inner diameter=218 mm, height=17 mm made of commercially available hard plaster (die-set plaster) over a period of 30 seconds, wherein the reducing level of slurry is topped up at regular intervals over three hours. After a further holding time of three hours in the mould, the shaped body billets are removed by adding purified compressed air into the air-extraction ducts of the mould. While the slurry is poured into the mould and while it is being held in the mould, an underpressure of 0.1 bar is maintained at the outer surface of the mould not wetted with the slurry, while the level of fluid of the slurry remains at normal atmospheric pressure.

The shaped body billet is then dried over 4 days at a temperature of 70° C. and then introduced into an oven. The oven is heated at a specified heating rate to the indicated temperature. The shaped body billet therein is then brought into the oven (data cf. table 23) for a specified holding time. Subsequently, the oven is switched off and left to cool without further cooling measures. The shaped body is then removed from the oven. The opaque quartz glass body formed in this way has the properties presented in table 23.

TABLE 23 Example L1-1 L3-11 L3-12 Prepared from quartz I1-1 I3-1 I3-1 glass grain Heating rate K/Min 5 2 5 Sinter temperature ° C. 1390 1400 1400 Holding time h 3 2.75 3 C content ppm <4 <4 <4 Cl content ppm 100-200 <60 <60 Al content ppb 20 20 20 Total of the concentrations of ppb <1000 <1000 <1000 Ca, Co, Cr, Cu, Fe, Ge, Hf, K, Li, Mg, Mn, Mo, Na, Nb, Ni, Ti, V, W, Zn, Zr Viscosity @1250° C. Lg(η/dPas) 12.09 ± 0.15 12.51 ± 0.2  12.54 ± 0.2  @1300° C. 11.41 ± 0.1  11.62 ± 0.12 11.60 ± 0.12 @1350° C. 10.80 ± 0.07 11.03 ± 0.06 11.07 ± 0.06 Mean density g/cm³ 2.162 2.170 2.153 Hemispherical Reflection % >75 >75 >80 350-2000 nm* Direct transmission % <0.5 <0.5 <0.3 350-2000 nm *measured with Perkin Elmer Lambda 2000 Spectrometer with integrating sphere fixture

M. Preparation of an Opaque Body II

The previously prepared quartz glass grain I4-1 is introduced into a rotating cylindrical steel mould with a sheet thickness of 30 mm. Subsequently, an electric arc is ignited and the poured sheet left to melt on a wall thickness of ca. 20 mm, glassy. A sealed, glassy sheet is produced which still contains a residual bubble content of <10%. The quartz glass body formed in this way has the properties presented in table 24.

TABLE 24 Example M1-1 M1-22 Prepared from quartz I1-3 I4-1 glass grain C content ppm <4 <4 Cl content ppm 100-200 <60 Al content ppb 20 20 Total of the concentrations ppb <1000 <1000 of Ca, Co, Cr, Cu, Fe, Ge, Hf, K, Li, Mg, Mn, Mo, Na, Nb, Ni, Ti, V, W, Zn, Zr Viscosity @1250° C. Lg(η/dPas) 11.21 ± 0.15 12.57 ± 0.2  @1300° C. 10.62 ± 0.1  11.12 ± 0.12 @1350° C. 10.10 ± 0.07 10.82 ± 0.06 Mean density g/cm³ 2.112 2.151 Hemispherical reflection % >60 >50 350-2000 nm* Direct transmission % <3 <8 350-2000 nm *measured with Perkin Elmer Lambda 2000 Spectrometer with integrating sphere fixture 

1-14. (canceled)
 15. A process for the preparation of a quartz glass grain comprising: providing a silicon dioxide granulate, wherein the silicon dioxide granulate has been prepared from pyrogenically produced silicon dioxide powder; making a glass melt out of the silicon dioxide granulate in a melting crucible; making a quartz glass body out of at least part of the glass melt; and reducing the size of the quartz glass body to obtain the quartz glass grain.
 16. The process according to claim 15, wherein the melting crucible has at least one inlet and an outlet and wherein the inlet is arranged above the outlet.
 17. The process according to claim 15, wherein the reduction in size of the quartz glass body takes place by high voltage discharge pulses.
 18. The process according to claim 15, wherein the quartz glass grain comprises at least one of: an OH content of less than 500 ppm; a chlorine content of less than 60 ppm; an aluminium content of less than 200 ppb; a BET surface area of less than 1 m²/g; a bulk density in a range from 1.1 to 1.4 g/cm³; a particle size D₅₀ for melting in a range from 50 to 500 μm; a particle size D₅₀ for the slurry in a range from 0.5 to 5 μm; a metal content of metals different to aluminium of less than 2 ppm; and viscosity (η=1013 hPa) in a range from log₁₀ (η(1250° C.)/dPas)=11.4 to log₁₀ (η(1250° C.)/dPas)=12.9 or log₁₀ (η(1300° C.)/dPas)=11.1 to log₁₀ (η(1300° C.)/dPas)=12.2 or log₁₀ (η(1350° C.)/dPas)=10.5 to log₁₀ (η(1350° C.)/dPas)=11.5; wherein the ppm and ppb are each based on the total weight of the quartz glass grain.
 19. The process according to claim 15, wherein melt energy in making a glass melt out of the silicon dioxide granulate is transferred to the silicon dioxide granulate via a solid surface.
 20. The process according to claim 15, wherein the silicon dioxide granulate comprises at least one of: a BET surface area in a range from 20 to 50 m²/g; a mean particle size in a range from 50 to 500 μm; a bulk density in a range from 0.5 to 1.2 g/cm³; a carbon content of less than 50 ppm; an aluminium content of less than 200 ppb; a tamped density in a range from 0.7 to 1.0 g/cm³; a pore volume in a range from 0.1 to 2.5 mL/g; an angle of repose in a range from 23 to 26° ; a particle size distribution D₁₀ in a range from 50 to 150 μm; a particle size distribution D₅₀ in a range from 150 to 300 μm; and a particle size distribution D90 in a range from 250 to 620 μm, wherein the ppm and ppb are each based on the total weight of the silicon dioxide granulate.
 21. The process according to claim 15, wherein the silicon dioxide powder is prepared from a compound selected from the group consisting of siloxanes, silicon alkoxides and silicon halides.
 22. A quartz glass grain obtained by a process according to claim
 15. 23. The quartz glass grain according to claim 22, comprising at least one of: an OH content of less than 500 ppm; a chlorine content of less than 60 ppm; an aluminium content of less than 200 ppb; a BET surface area of less than 1 m²/g; a bulk density in a range from 1.1 to 1.4 g/cm³; a particle size D₅₀ for melting in a range from 50 to 500 μm; a particle size D₅₀ for the slurry in a range from 0.5 to 5 mm; a metal content of metals different to aluminium of less than 2 ppm; and viscosity (p=1013 hPa) in a range from log₁₀ (η(1250° C.)/dPas)=11.4 to log₁₀ (η(1250° C.)/dPas)=12.9 or log₁₀ (η(1300° C.)/dPas)=11.1 to log₁₀ (η(1300° C.)/dPas)=12.2 or log₁₀ (η(1350° C.)/dPas)=10.5 to log₁₀ (η(1350° C.)/dPas)=11.5; wherein the ppm and ppb are each based on the total weight of the quartz glass grain.
 24. A process for the preparation of an opaque formed body comprising: providing a quartz glass grain according to claim 22; and processing the quartz glass grain to an opaque formed body.
 25. The process according to claim 24, wherein the process comprises a sintering.
 26. An opaque formed body obtained by a process according to claim
 24. 27. The opaque formed body according to claim 26, wherein the opaque formed body comprises at least one of: an opacity of more than 5; a BET surface area of less than 1 m²/g; a cylindrical form; a sheet; an OH content of less than 500 ppm; a chlorine content of less than 60 ppm; an aluminium content of less than 200 ppb; an ODC content of less than 5*10¹⁸/cm³; a carbon content of less than 5 ppm; and a total metal content of metals different to aluminium of less than 2 ppm; wherein the ppm and ppb are each based on the total weight of the formed body.
 28. A use of a quartz glass grain for preparing opaque products comprising quartz glass, wherein the quartz glass grain comprises: an OH content of less than 500 ppm; a chlorine content of less than 60 ppm; and an aluminium content of less than 200 ppb; wherein the ppm and ppb are each based on the total weight of the quartz glass grain. 